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Rotation of Axes
Identify nondegenerate conic sections given their general form equations.
Rotation of Axes
- Using rotation of axes formulas.
- Identify conic sections by their equations. (IA 11.4.3)
We can identify a conic from its equations by looking at the signs and coefficients of the variables that are squared.
| Conic | Characteristics of \({x}^{2}\text{-}\) and \({y}^{2}\text{-}\) terms | Example |
| Parabola | Either \({x}^{2}\) OR \({y}^{2}.\) Only one variable is squared. | \(x=3{y}^{2}-2y+1\) |
| Circle | \({x}^{2}\text{-}\) and \({y}^{2}\text{-}\) terms have the same coefficients | \({x}^{2}+{y}^{2}=49\) |
| Ellipse | \({x}^{2}\text{-}\) and \({y}^{2}\text{-}\) terms have the same sign, different coefficients | \(4{x}^{2}+25{y}^{2}=100\) |
| Hyperbola | \({x}^{2}\text{-}\) and \({y}^{2}\text{-}\) terms have different signs, different coefficients | \(25{y}^{2}-4{x}^{2}=100\) |
Example
Try it.
- ⓐ \(x=-{y}^{2}-2y+3\)
- ⓑ \(9{y}^{2}-{x}^{2}+18y-4x-4=0\)
- ⓒ \(9{x}^{2}+25{y}^{2}=225\)
- ⓓ \({x}^{2}+{y}^{2}-4x+10y-7=0\)
Solution
- ⓐ \(x=-{y}^{2}-2y+3\)
Parabola: only one variable is squared. - ⓑ \(9{y}^{2}-{x}^{2}+18y-4x-4=0\)
Hyperbola: \({x}^{2}\) and \({y}^{2}\) have different signs and different coefficients. - ⓒ \(9{x}^{2}+25{y}^{2}=225\)
Ellipse: \({x}^{2}\) and \({y}^{2}\) have the same signs and different coefficients. - ⓓ \({x}^{2}+{y}^{2}-4x+10y-7=0\)
Circle: \({x}^{2}\) and \({y}^{2}\) have the same signs and the same signs coefficients.
Identify conic sections by their equations.
Try it.
\(x=-2{y}^{2}-12y-16\)
Try it.
\({x}^{2}+{y}^{2}=9\)
Try it.
\(16{x}^{2}-4{y}^{2}+64x-24y-36=0\)
Try it.
\(16{x}^{2}+36{y}^{2}=576\)
Condensed — the full section is in OpenStax Precalculus 2e.
Identifying Nondegenerate Conics in General Form
In previous sections of this chapter, we have focused on the standard form equations for nondegenerate conic sections. In this section, we will shift our focus to the general form equation, which can be used for any conic. The general form is set equal to zero, and the terms and coefficients are given in a particular order, as shown below.
\[A{x}^{2}+Bxy+C{y}^{2}+Dx+Ey+F=0\]where \(A,B,\) and \(C\) are not all zero. We can use the values of the coefficients to identify which type conic is represented by a given equation.
You may notice that the general form equation has an \(xy\) term that we have not seen in any of the standard form equations. As we will discuss later, the \(xy\) term rotates the conic whenever \(B\) is not equal to zero.
| Conic Sections | Example |
| ellipse | \(4{x}^{2}+9{y}^{2}=1\) |
| circle | \(4{x}^{2}+4{y}^{2}=1\) |
| hyperbola | \(4{x}^{2}-9{y}^{2}=1\) |
| parabola | \(4{x}^{2}=9y\ \text{or }4{y}^{2}=9x\) |
| one line | \(4x+9y=1\) |
| intersecting lines | \((x-4)(y+4)=0\) |
| parallel lines | \((x-4)(x-9)=0\) |
| a point | \(4{x}^{2}+4{y}^{2}=0\) |
| no graph | \(4{x}^{2}+4{y}^{2}=\ -\ 1\) |
Example
Try it.
Identify the graph of each of the following nondegenerate conic sections.
- ⓐ\(4{x}^{2}-9{y}^{2}+36x+36y-125=0\)
- ⓑ \(9{y}^{2}+16x+36y-10=0\)
- ⓒ \(3{x}^{2}+3{y}^{2}-2x-6y-4=0\)
- ⓓ \(-25{x}^{2}-4{y}^{2}+100x+16y+20=0\)
Solution
- ⓐ Rewriting the general form, we have
\(A=4\) and \(C=-9,\) so we observe that \(A\) and \(C\) have opposite signs. The graph of this equation is a hyperbola.
- ⓑ Rewriting the general form, we have
\(A=0\) and \(C=9.\) We can determine that the equation is a parabola, since \(A\) is zero.
- ⓒ Rewriting the general form, we have
\(A=3\) and \(C=3.\) Because \(A=C,\) the graph of this equation is a circle.
- ⓓ Rewriting the general form, we have
\(A=-25\) and \(C=-4.\) Because \(AC>0\) and \(A\ne C,\) the graph of this equation is an ellipse.
Condensed — the full section is in OpenStax Precalculus 2e.
Writing Equations of Rotated Conics in Standard Form
Now that we can find the standard form of a conic when we are given an angle of rotation, we will learn how to transform the equation of a conic given in the form \(A{x}^{2}+Bxy+C{y}^{2}+Dx+Ey+F=0\) into standard form by rotating the axes. To do so, we will rewrite the general form as an equation in the \({x}^{'}\) and \({y}^{'}\) coordinate system without the \({x}^{'}{y}^{'}\) term, by rotating the axes by a measure of \(\theta\) that satisfies
\[\text{cot}(2\theta )=\frac{A-C}{B}\]We have learned already that any conic may be represented by the second degree equation
\[A{x}^{2}+Bxy+C{y}^{2}+Dx+Ey+F=0\]where \(A,B,\) and \(C\) are not all zero. However, if \(B\ne 0,\) then we have an \(xy\) term that prevents us from rewriting the equation in standard form. To eliminate it, we can rotate the axes by an acute angle \(\theta\) where \(\text{cot}(2\theta )=\frac{A-C}{B}.\)
- If \(\text{cot}(2\theta )>0,\) then \(2\theta\) is in the first quadrant, and \(\theta\) is between \((0^{\circ},45^{\circ}).\)
- If \(\text{cot}(2\theta )<0,\) then \(2\theta\) is in the second quadrant, and \(\theta\) is between \((45^{\circ},90^{\circ}).\)
- If \(A=C,\) then \(\theta =45^{\circ}.\)
Condensed — the full section is in OpenStax Precalculus 2e.
Identifying Conics without Rotating Axes
Now we have come full circle. How do we identify the type of conic described by an equation? What happens when the axes are rotated? Recall, the general form of a conic is
\[A{x}^{2}+Bxy+C{y}^{2}+Dx+Ey+F=0\]If we apply the rotation formulas to this equation we get the form
\[{A}^{'}{{x}^{'}}^{2}+{B}^{'}{x}^{'}{y}^{'}+{C}^{'}{{y}^{'}}^{2}+{D}^{'}{x}^{'}+{E}^{'}{y}^{'}+{F}^{'}=0\]It may be shown that \({B}^{2}-4AC={{B}^{'}}^{2}-4{A}^{'}{C}^{'}.\) The expression does not vary after rotation, so we call the expression invariant. The discriminant, \({B}^{2}-4AC,\) is invariant and remains unchanged after rotation. Because the discriminant remains unchanged, observing the discriminant enables us to identify the conic section.
Example
Try it.
Identify the conic for each of the following without rotating axes.
- ⓐ \(5{x}^{2}+2\sqrt{3}xy+2{y}^{2}-5=0\)
- ⓑ \(5{x}^{2}+2\sqrt{3}xy+12{y}^{2}-5=0\)
Solution
- ⓐ Let’s begin by determining \(A,B,\) and \(C.\)
\[\underset{A}{\underset{︸}{5}}{x}^{2}+\underset{B}{\underset{︸}{2\sqrt{3}}}xy+\underset{C}{\underset{︸}{2}}{y}^{2}-5=0\]
Now, we find the discriminant.
\[\begin{array}{l}{B}^{2}-4AC={(2\sqrt{3})}^{2}-4(5)(2) \\ \ =4(3)-40 \\ \ =12-40 \\ \ =-28<0\end{array}\]Therefore, \(5{x}^{2}+2\sqrt{3}xy+2{y}^{2}-5=0\) represents an ellipse.
- ⓑ Again, let’s begin by determining \(A,B,\) and \(C.\)
\[\underset{A}{\underset{︸}{5}}{x}^{2}+\underset{B}{\underset{︸}{2\sqrt{3}}}xy+\underset{C}{\underset{︸}{12}}{y}^{2}-5=0\]
Now, we find the discriminant.
\[\begin{array}{l}{B}^{2}-4AC={(2\sqrt{3})}^{2}-4(5)(12) \\ \ =4(3)-240 \\ \ =12-240 \\ \ =-228<0\end{array}\]Therefore, \(5{x}^{2}+2\sqrt{3}xy+12{y}^{2}-5=0\) represents an ellipse.
Key Equations
| General Form equation of a conic section | \(A{x}^{2}+Bxy+C{y}^{2}+Dx+Ey+F=0\) |
| Rotation of a conic section | \[\begin{array}{l}x={x}^{'}\cos \ \theta -{y}^{'}\sin \ \theta \\ y={x}^{'}\sin \ \theta +{y}^{'}\cos \ \theta \end{array}\] |
| Angle of rotation | \(\theta ,\text{where }\text{cot}(2\theta )=\frac{A-C}{B}\) |
Key Concepts
- Four basic shapes can result from the intersection of a plane with a pair of right circular cones connected tail to tail. They include an ellipse, a circle, a hyperbola, and a parabola.
- A nondegenerate conic section has the general form \(A{x}^{2}+Bxy+C{y}^{2}+Dx+Ey+F=0\) where \(A,B\) and \(C\) are not all zero. The values of \(A,B,\) and \(C\) determine the type of conic. See .
- Equations of conic sections with an \(xy\) term have been rotated about the origin. See .
- The general form can be transformed into an equation in the \({x}^{'}\) and \({y}^{'}\) coordinate system without the \({x}^{'}{y}^{'}\) term. See and .
- An expression is described as invariant if it remains unchanged after rotating. Because the discriminant is invariant, observing it enables us to identify the conic section. See .
Practice (40)
Try each one on paper first. Reveal the answer to check; verified ones can be opened in the solver for every step.
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Find a new representation of the given equation after rotating through the given angle.
\(3{x}^{2}+xy+3{y}^{2}-5=0,\theta =45º\)
જવાબ બતાવો
Find x and y using the rotation of axes formulas, substitute θ=45º. \(\left\{\begin{array}{l}x=x'\text{cos}\theta -y'\text{sin}\theta \\ y=x'\text{sin}\theta +y'\text{cos}\theta \end{array}\right\) \(x=x'\left(\frac{1}{\sqrt{2}}\right)-y'\left(\frac{1}{\sqrt{2}}\right)\)
\(x=\frac{x'-y'}{\sqrt{2}}\)\(y=x'\left(\frac{1}{\sqrt{2}}\right)-y'\left(\frac{1}{\sqrt{2}}\right)\)
\(y=\frac{x'-y'}{\sqrt{2}}\)\(x=x'\left(\frac{1}{\sqrt{2}}\right)-y'\left(\frac{1}{\sqrt{2}}\right)\)
\(x=\frac{x'-y'}{\sqrt{2}}\)\(y=x'\left(\frac{1}{\sqrt{2}}\right)-y'\left(\frac{1}{\sqrt{2}}\right)\)
\(y=\frac{x'-y'}{\sqrt{2}}\)Substitute the expressions for x and y into the given equation and simplify. \(3{x}^{2}+xy+3{y}^{2}-5=0\) \(3{\left(\frac{x'-y'}{\sqrt{2}}\right)}^{2}+\left(\frac{x'-y'}{\sqrt{2}}\right)\left(\frac{x'-y'}{\sqrt{2}}\right)+3{\left(\frac{x'-y'}{\sqrt{2}}\right)}^{2}-5=0\) Foil each term. \(3(\frac{{x'}^{2}-2x'y'+{y'}^{2}}{2})2+\frac{{x'}^{2}-{y'}^{2}}{2}+3(\frac{{x'}^{2}-2x'y'+{y'}^{2}}{2})-5=0\) Multiply by 2 to get rid of the fraction. \(3({x'}^{2}-2x'y'+{y'}^{2})2+{x'}^{2}-{y'}^{2}+3({x'}^{2}-2x'y'+{y'}^{2})-10=0\) Combine like terms. \(3{x'}^{2}-6x'y'+3{y'}^{2}+{x'}^{2}-{y'}^{2}+3{x'}^{2}+6x'y'+3{y'}^{2}-10=0\ 7{x'}^{2}+{5y'}^{2}-10=0\ 7{x'}^{2}+{5y'}^{2}=10\) Write the equations with x′ and y′ in standard form. Set equal to 1.
\(\frac{7{x'}^{2}}{10}+\frac{{5y'}^{2}}{10}=1\ \frac{{x'}^{2}}{\frac{10}{7}}+\frac{{y'}^{2}}{2}=1\\) -
Find a new representation of the given equation after rotating through the given angle. Use the steps outlined to assist you in your work.
\(4{x}^{2}-xy+4{y}^{2}-2=0,\theta =45º\)
Find x and y using the rotation of axes formulas, substitute θ=45º. Substitute the expressions for x and y into the given equation and simplify. Write the equations with x′ and y′ in standard form. -
- ⓐ \(x=-{y}^{2}-2y+3\)
- ⓑ \(9{y}^{2}-{x}^{2}+18y-4x-4=0\)
- ⓒ \(9{x}^{2}+25{y}^{2}=225\)
- ⓓ \({x}^{2}+{y}^{2}-4x+10y-7=0\)
જવાબ બતાવો
- ⓐ \(x=-{y}^{2}-2y+3\)
Parabola: only one variable is squared. - ⓑ \(9{y}^{2}-{x}^{2}+18y-4x-4=0\)
Hyperbola: \({x}^{2}\) and \({y}^{2}\) have different signs and different coefficients. - ⓒ \(9{x}^{2}+25{y}^{2}=225\)
Ellipse: \({x}^{2}\) and \({y}^{2}\) have the same signs and different coefficients. - ⓓ \({x}^{2}+{y}^{2}-4x+10y-7=0\)
Circle: \({x}^{2}\) and \({y}^{2}\) have the same signs and the same signs coefficients.
-
\(x=-2{y}^{2}-12y-16\)
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\({x}^{2}+{y}^{2}=9\)
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\(16{x}^{2}-4{y}^{2}+64x-24y-36=0\)
-
\(16{x}^{2}+36{y}^{2}=576\)
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Identify the graph of each of the following nondegenerate conic sections.
- ⓐ\(4{x}^{2}-9{y}^{2}+36x+36y-125=0\)
- ⓑ \(9{y}^{2}+16x+36y-10=0\)
- ⓒ \(3{x}^{2}+3{y}^{2}-2x-6y-4=0\)
- ⓓ \(-25{x}^{2}-4{y}^{2}+100x+16y+20=0\)
જવાબ બતાવો
- ⓐ Rewriting the general form, we have
\(A=4\) and \(C=-9,\) so we observe that \(A\) and \(C\) have opposite signs. The graph of this equation is a hyperbola.
- ⓑ Rewriting the general form, we have
\(A=0\) and \(C=9.\) We can determine that the equation is a parabola, since \(A\) is zero.
- ⓒ Rewriting the general form, we have
\(A=3\) and \(C=3.\) Because \(A=C,\) the graph of this equation is a circle.
- ⓓ Rewriting the general form, we have
\(A=-25\) and \(C=-4.\) Because \(AC>0\) and \(A\ne C,\) the graph of this equation is an ellipse.
-
Identify the graph of each of the following nondegenerate conic sections.
- ⓐ \(16{y}^{2}-{x}^{2}+x-4y-9=0\)
- ⓑ \(16{x}^{2}+4{y}^{2}+16x+49y-81=0\)
જવાબ બતાવો
- ⓐ hyperbola
- ⓑ ellipse
-
Find a new representation of the equation \(2{x}^{2}-xy+2{y}^{2}-30=0\) after rotating through an angle of \(\theta =45^{\circ}.\)
જવાબ બતાવો
Find \(x\) and \(y,\) where \(x={x}^{'}\cos \ \theta -{y}^{'}\sin \ \theta\) and \(y={x}^{'}\sin \ \theta +{y}^{'}\cos \ \theta .\)
Because \(\theta =45^{\circ},\)
\[\begin{array}{l} \\ x={x}^{'}\cos (45^{\circ})-{y}^{'}\sin (45^{\circ}) \\ x={x}^{'}(\frac{1}{\sqrt{2}})-{y}^{'}(\frac{1}{\sqrt{2}}) \\ x=\frac{{x}^{'}-{y}^{'}}{\sqrt{2}}\end{array}\]and
\[\begin{array}{l} \\ \begin{array}{l}y={x}^{'}\sin (45^{\circ})+{y}^{'}\cos (45^{\circ}) \\ y={x}^{'}(\frac{1}{\sqrt{2}})+{y}^{'}(\frac{1}{\sqrt{2}}) \\ y=\frac{{x}^{'}+{y}^{'}}{\sqrt{2}}\end{array}\end{array}\]Substitute \(x={x}^{'}\cos \theta -{y}^{'}\sin \theta\) and \(y={x}^{'}\sin \ \theta +{y}^{'}\cos \ \theta\) into \(2{x}^{2}-xy+2{y}^{2}-30=0.\)
\[2{(\frac{{x}^{'}-{y}^{'}}{\sqrt{2}})}^{2}-(\frac{{x}^{'}-{y}^{'}}{\sqrt{2}})(\frac{{x}^{'}+{y}^{'}}{\sqrt{2}})+2{(\frac{{x}^{'}+{y}^{'}}{\sqrt{2}})}^{2}-30=0\]Simplify.
\[\begin{array}{lllll}2\frac{({x}^{'}-{y}^{'})({x}^{'}-{y}^{'})}{2}-\frac{({x}^{'}-{y}^{'})({x}^{'}+{y}^{'})}{2}+2\frac{({x}^{'}+{y}^{'})({x}^{'}+{y}^{'})}{2}-30=0 & \begin{array}{llll} & & & \end{array}\text{FOIL method} \\ \ {x}^{'}{}^{2}{-2{x}^{'}y}^{'}+{y}^{'}{}^{2}-\frac{({x}^{'}{}^{2}-{y}^{'}{}^{2})}{2}+{x}^{'}{}^{2}+2{x}^{'}{y}^{'}+{y}^{'}{}^{2}-30=0 & \begin{array}{llll} & & & \end{array}\text{Combine like terms}. \\ \ 2{x}^{'}{}^{2}+2{y}^{'}{}^{2}-\frac{({x}^{'}{}^{2}-{y}^{'}{}^{2})}{2}=30 & \begin{array}{llll} & & & \end{array}\text{Combine like terms}. \\ \ 2(2{x}^{'}{}^{2}+2{y}^{'}{}^{2}-\frac{({x}^{'}{}^{2}-{y}^{'}{}^{2})}{2})=2(30) & \begin{array}{llll} & & & \end{array}\text{Multiply both sides by 2}. \\ \ 4{x}^{'}{}^{2}+4{y}^{'}{}^{2}-({x}^{'}{}^{2}-{y}^{'}{}^{2})=60 & \begin{array}{llll} & & & \end{array}\text{Simplify}. \\ \ 4{x}^{'}{}^{2}+4{y}^{'}{}^{2}-{x}^{'}{}^{2}+{y}^{'}{}^{2}=60 & \begin{array}{llll} & & & \end{array}\text{Distribute}. \\ \ \frac{3{x}^{'}{}^{2}}{60}+\frac{5{y}^{'}{}^{2}}{60}=\frac{60}{60} & \begin{array}{llll} & & & \end{array}\text{Set equal to 1}.\end{array}\]Write the equations with \({x}^{'}\) and \({y}^{'}\) in the standard form.
\[\frac{{{x}^{'}}^{2}}{20}+\frac{{{y}^{'}}^{2}}{12}=1\]This equation is an ellipse. shows the graph.
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Rewrite the equation \(8{x}^{2}-12xy+17{y}^{2}=20\) in the \({x}^{'}{y}^{'}\) system without an \({x}^{'}{y}^{'}\) term.
જવાબ બતાવો
First, we find \(\text{cot}(2\theta ).\) See .
\[\begin{array}{l}8{x}^{2}-12xy+17{y}^{2}=20⇒A=8,\ B=-12\ \text{and}\ C=17 \\ \ \ \text{cot}(2\theta )=\frac{A-C}{B}=\frac{8-17}{-12} \\ \ \ \text{cot}(2\theta )=\frac{-9}{-12}=\frac{3}{4}\end{array}\]\[\text{cot}(2\theta )=\frac{3}{4}=\frac{\text{adjacent}}{\text{opposite}}\]So the hypotenuse is
\[\begin{array}{l}{3}^{2}+{4}^{2}={h}^{2} \\ 9+16={h}^{2} \\ 25={h}^{2} \\ h=5\ \ \end{array}\]Next, we find \(\sin \ \theta\) and \(\cos \ \theta .\)
\[\begin{array}{l}\begin{array}{l} \\ \\ \sin \ \theta =\sqrt{\frac{1-\cos (2\theta )}{2}}=\sqrt{\frac{1-\frac{3}{5}}{2}}=\sqrt{\frac{\frac{5}{5}-\frac{3}{5}}{2}}=\sqrt{\frac{5-3}{5}⋅\frac{1}{2}}=\sqrt{\frac{2}{10}}=\sqrt{\frac{1}{5}}\end{array} \\ \sin \ \theta =\frac{1}{\sqrt{5}} \\ \cos \ \theta =\sqrt{\frac{1+\cos (2\theta )}{2}}=\sqrt{\frac{1+\frac{3}{5}}{2}}=\sqrt{\frac{\frac{5}{5}+\frac{3}{5}}{2}}=\sqrt{\frac{5+3}{5}⋅\frac{1}{2}}=\sqrt{\frac{8}{10}}=\sqrt{\frac{4}{5}} \\ \cos \ \theta =\frac{2}{\sqrt{5}}\end{array}\]Substitute the values of \(\sin \ \theta\) and \(\cos \ \theta\) into \(x={x}^{'}\cos \ \theta -{y}^{'}\sin \ \theta\) and \(y={x}^{'}\sin \ \theta +{y}^{'}\cos \ \theta .\)
\[\begin{array}{l} \\ \begin{array}{l}x={x}^{'}\cos \ \theta -{y}^{'}\sin \ \theta \\ x={x}^{'}(\frac{2}{\sqrt{5}})-{y}^{'}(\frac{1}{\sqrt{5}}) \\ x=\frac{2{x}^{'}-{y}^{'}}{\sqrt{5}}\end{array}\end{array}\]and
\[\begin{array}{l}\begin{array}{l} \\ y={x}^{'}\sin \ \theta +{y}^{'}\cos \ \theta \end{array} \\ y={x}^{'}(\frac{1}{\sqrt{5}})+{y}^{'}(\frac{2}{\sqrt{5}}) \\ y=\frac{{x}^{'}+2{y}^{'}}{\sqrt{5}}\end{array}\]Substitute the expressions for \(x\) and \(y\) into in the given equation, and then simplify.
\[\begin{array}{l}\ 8{(\frac{2{x}^{'}-{y}^{'}}{\sqrt{5}})}^{2}-12(\frac{2{x}^{'}-{y}^{'}}{\sqrt{5}})(\frac{{x}^{'}+2{y}^{'}}{\sqrt{5}})+17{(\frac{{x}^{'}+2{y}^{'}}{\sqrt{5}})}^{2}=20\ \\ \ 8(\frac{(2{x}^{'}-{y}^{'})(2{x}^{'}-{y}^{'})}{5})-12(\frac{(2{x}^{'}-{y}^{'})({x}^{'}+2{y}^{'})}{5})+17(\frac{({x}^{'}+2{y}^{'})({x}^{'}+2{y}^{'})}{5})=20\ \\ \ 8(4{x}^{'}{}^{2}-4{x}^{'}{y}^{'}+{y}^{'}{}^{2})-12(2{x}^{'}{}^{2}+3{x}^{'}{y}^{'}-2{y}^{'}{}^{2})+17({x}^{'}{}^{2}+4{x}^{'}{y}^{'}+4{y}^{'}{}^{2})=100 \\ 32{x}^{'}{}^{2}-32{x}^{'}{y}^{'}+8{y}^{'}{}^{2}-24{x}^{'}{}^{2}-36{x}^{'}{y}^{'}+24{y}^{'}{}^{2}+17{x}^{'}{}^{2}+68{x}^{'}{y}^{'}+68{y}^{'}{}^{2}=100 \\ \ 25{x}^{'}{}^{2}+100{y}^{'}{}^{2}=100\ \\ \ \frac{25}{100}{x}^{'}{}^{2}+\frac{100}{100}{y}^{'}{}^{2}=\frac{100}{100}\end{array}\]Write the equations with \({x}^{'}\) and \({y}^{'}\) in the standard form with respect to the new coordinate system.
\[\frac{{{x}^{'}}^{2}}{4}+\frac{{{y}^{'}}^{2}}{1}=1\]shows the graph of the ellipse.
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Rewrite the \(13{x}^{2}-6\sqrt{3}xy+7{y}^{2}=16\) in the \({x}^{'}{y}^{'}\) system without the \({x}^{'}{y}^{'}\) term.
જવાબ બતાવો
\(\frac{{{x}^{'}}^{2}}{4}+\frac{{{y}^{'}}^{2}}{1}=1\)
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Graph the following equation relative to the \({x}^{'}{y}^{'}\) system:
\[{x}^{2}+12xy-4{y}^{2}=30\]જવાબ બતાવો
First, we find \(\text{cot}(2\theta ).\)
\[{x}^{2}+12xy-4{y}^{2}=20⇒A=1,\ B=12,\text{and }C=-4\]\[\begin{array}{l}\text{cot}(2\theta )=\frac{A-C}{B} \\ \text{cot}(2\theta )=\frac{1-(-4)}{12} \\ \text{cot}(2\theta )=\frac{5}{12}\end{array}\]Because \(\text{cot}(2\theta )=\frac{5}{12},\) we can draw a reference triangle as in .
\[\text{cot}(2\theta )=\frac{5}{12}=\frac{\text{adjacent}}{\text{opposite}}\]Thus, the hypotenuse is
\[\begin{array}{l}{5}^{2}+{12}^{2}={h}^{2} \\ 25+144={h}^{2} \\ 169={h}^{2} \\ h=13\end{array}\]Next, we find \(\sin \ \theta\) and \(\cos \ \theta .\) We will use half-angle identities.
\[\begin{array}{l}\begin{array}{l} \\ \\ \sin \ \theta =\sqrt{\frac{1-\cos (2\theta )}{2}}=\sqrt{\frac{1-\frac{5}{13}}{2}}=\sqrt{\frac{\frac{13}{13}-\frac{5}{13}}{2}}=\sqrt{\frac{8}{13}⋅\frac{1}{2}}=\frac{2}{\sqrt{13}}\end{array} \\ \cos \ \theta =\sqrt{\frac{1+\cos (2\theta )}{2}}=\sqrt{\frac{1+\frac{5}{13}}{2}}=\sqrt{\frac{\frac{13}{13}+\frac{5}{13}}{2}}=\sqrt{\frac{18}{13}⋅\frac{1}{2}}=\frac{3}{\sqrt{13}}\end{array}\]Now we find \(x\) and \(y\text{.\,}\)
\[\begin{array}{l} \\ x={x}^{'}\cos \ \theta -{y}^{'}\sin \ \theta \\ x={x}^{'}(\frac{3}{\sqrt{13}})-{y}^{'}(\frac{2}{\sqrt{13}}) \\ x=\frac{3{x}^{'}-2{y}^{'}}{\sqrt{13}}\end{array}\]and
\[\begin{array}{l} \\ y={x}^{'}\sin \ \theta +{y}^{'}\cos \ \theta \\ y={x}^{'}(\frac{2}{\sqrt{13}})+{y}^{'}(\frac{3}{\sqrt{13}}) \\ y=\frac{2{x}^{'}+3{y}^{'}}{\sqrt{13}}\end{array}\]Now we substitute \(x=\frac{3{x}^{'}-2{y}^{'}}{\sqrt{13}}\) and \(y=\frac{2{x}^{'}+3{y}^{'}}{\sqrt{13}}\) into \({x}^{2}+12xy-4{y}^{2}=30.\)
\[\begin{array}{llll}\ {(\frac{3{x}^{'}-2{y}^{'}}{\sqrt{13}})}^{2}+12(\frac{3{x}^{'}-2{y}^{'}}{\sqrt{13}})(\frac{2{x}^{'}+3{y}^{'}}{\sqrt{13}})-4{(\frac{2{x}^{'}+3{y}^{'}}{\sqrt{13}})}^{2}=30 & & & \\ \ \ (\frac{1}{13})[{(3{x}^{'}-2{y}^{'})}^{2}+12(3{x}^{'}-2{y}^{'})(2{x}^{'}+3{y}^{'})-4{(2{x}^{'}+3{y}^{'})}^{2}]=30 & & & \text{Factor}. \\ (\frac{1}{13})[9{x}^{'}{}^{2}-12{x}^{'}{y}^{'}+4{y}^{'}{}^{2}+12(6{x}^{'}{}^{2}+5{x}^{'}{y}^{'}-6{y}^{'}{}^{2})-4(4{x}^{'}{}^{2}+12{x}^{'}{y}^{'}+9{y}^{'}{}^{2})]=30 & & & \text{Multiply}. \\ \ (\frac{1}{13})[9{x}^{'}{}^{2}-12{x}^{'}{y}^{'}+4{y}^{'}{}^{2}+72{x}^{'}{}^{2}+60{x}^{'}{y}^{'}-72{y}^{'}{}^{2}-16{x}^{'}{}^{2}-48{x}^{'}{y}^{'}-36{y}^{'}{}^{2}]=30 & & & \text{Distribute}. \\ \ \ (\frac{1}{13})[65{x}^{'}{}^{2}-104{y}^{'}{}^{2}]=30 & & & \text{Combine like terms}. \\ \ 65{x}^{'}{}^{2}-104{y}^{'}{}^{2}=390 & & & \text{Multiply}.\ \\ \ \frac{{x}^{'}{}^{2}}{6}-\frac{4{y}^{'}{}^{2}}{15}=1 & & & \text{Divide by 390}.\end{array}\]shows the graph of the hyperbola \(\frac{{{x}^{'}}^{2}}{6}-\frac{4{{y}^{'}}^{2}}{15}=1.\ \\)
-
Identify the conic for each of the following without rotating axes.
- ⓐ \(5{x}^{2}+2\sqrt{3}xy+2{y}^{2}-5=0\)
- ⓑ \(5{x}^{2}+2\sqrt{3}xy+12{y}^{2}-5=0\)
જવાબ બતાવો
- ⓐ Let’s begin by determining \(A,B,\) and \(C.\)
\[\underset{A}{\underset{︸}{5}}{x}^{2}+\underset{B}{\underset{︸}{2\sqrt{3}}}xy+\underset{C}{\underset{︸}{2}}{y}^{2}-5=0\]
Now, we find the discriminant.
\[\begin{array}{l}{B}^{2}-4AC={(2\sqrt{3})}^{2}-4(5)(2) \\ \ =4(3)-40 \\ \ =12-40 \\ \ =-28<0\end{array}\]Therefore, \(5{x}^{2}+2\sqrt{3}xy+2{y}^{2}-5=0\) represents an ellipse.
- ⓑ Again, let’s begin by determining \(A,B,\) and \(C.\)
\[\underset{A}{\underset{︸}{5}}{x}^{2}+\underset{B}{\underset{︸}{2\sqrt{3}}}xy+\underset{C}{\underset{︸}{12}}{y}^{2}-5=0\]
Now, we find the discriminant.
\[\begin{array}{l}{B}^{2}-4AC={(2\sqrt{3})}^{2}-4(5)(12) \\ \ =4(3)-240 \\ \ =12-240 \\ \ =-228<0\end{array}\]Therefore, \(5{x}^{2}+2\sqrt{3}xy+12{y}^{2}-5=0\) represents an ellipse.
-
Identify the conic for each of the following without rotating axes.
- ⓐ \({x}^{2}-9xy+3{y}^{2}-12=0\)
- ⓑ \(10{x}^{2}-9xy+4{y}^{2}-4=0\)
જવાબ બતાવો
- ⓐ hyperbola
- ⓑ ellipse
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What effect does the \(xy\) term have on the graph of a conic section?
જવાબ બતાવો
The \(xy\) term causes a rotation of the graph to occur.
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If the equation of a conic section is written in the form \(A{x}^{2}+B{y}^{2}+Cx+Dy+E=0\) and \(AB=0,\) what can we conclude?
-
If the equation of a conic section is written in the form \(A{x}^{2}+Bxy+C{y}^{2}+Dx+Ey+F=0,\) and \({B}^{2}-4AC>0,\) what can we conclude?
જવાબ બતાવો
The conic section is a hyperbola.
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Given the equation \(a{x}^{2}+4x+3{y}^{2}-12=0,\) what can we conclude if \(a>0?\)
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For the equation \(A{x}^{2}+Bxy+C{y}^{2}+Dx+Ey+F=0,\) the value of \(\theta\) that satisfies \(\text{cot}(2\theta )=\frac{A-C}{B}\) gives us what information?
જવાબ બતાવો
It gives the angle of rotation of the axes in order to eliminate the \(xy\) term.
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\(9{x}^{2}+4{y}^{2}+72x+36y-500=0\)
-
\({x}^{2}-10x+4y-10=0\)
જવાબ બતાવો
\(AB=0,\) parabola
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\(2{x}^{2}-2{y}^{2}+4x-6y-2=0\)
-
\(4{x}^{2}-{y}^{2}+8x-1=0\)
જવાબ બતાવો
\(AB=-4<0,\) hyperbola
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\(4{y}^{2}-5x+9y+1=0\)
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\(2{x}^{2}+3{y}^{2}-8x-12y+2=0\)
જવાબ બતાવો
\(AB=6>0,\) ellipse
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\(4{x}^{2}+9xy+4{y}^{2}-36y-125=0\)
-
\(3{x}^{2}+6xy+3{y}^{2}-36y-125=0\)
જવાબ બતાવો
\({B}^{2}-4AC=0,\) parabola
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\(-3{x}^{2}+3\sqrt{3}xy-4{y}^{2}+9=0\)
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\(2{x}^{2}+4\sqrt{3}xy+6{y}^{2}-6x-3=0\)
જવાબ બતાવો
\({B}^{2}-4AC=0,\) parabola
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\(-{x}^{2}+4\sqrt{2}xy+2{y}^{2}-2y+1=0\)
-
\(8{x}^{2}+4\sqrt{2}xy+4{y}^{2}-10x+1=0\)
જવાબ બતાવો
\({B}^{2}-4AC=-96<0,\) ellipse
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\(3{x}^{2}+xy+3{y}^{2}-5=0,\theta =45^{\circ}\)
-
\(4{x}^{2}-xy+4{y}^{2}-2=0,\theta =45^{\circ}\)
જવાબ બતાવો
\(7{{x}^{'}}^{2}+9{{y}^{'}}^{2}-4=0\)
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\(2{x}^{2}+8xy-1=0,\theta =30^{\circ}\)
-
\(-2{x}^{2}+8xy+1=0,\theta =45^{\circ}\)
જવાબ બતાવો
\(3{{x}^{'}}^{2}+2{x}^{'}{y}^{'}-5{{y}^{'}}^{2}+1=0\)
-
\(4{x}^{2}+\sqrt{2}xy+4{y}^{2}+y+2=0,\theta =45^{\circ}\)
-
\({x}^{2}+3\sqrt{3}xy+4{y}^{2}+y-2=0\)
જવાબ બતાવો
\(\theta ={60}^{∘},11{{x}^{'}}^{2}-{{y}^{'}}^{2}+\sqrt{3}{x}^{'}+{y}^{'}-4=0\)
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\(4{x}^{2}+2\sqrt{3}xy+6{y}^{2}+y-2=0\)
-
\(9{x}^{2}-3\sqrt{3}xy+6{y}^{2}+4y-3=0\)
જવાબ બતાવો
\(\theta ={-30}^{∘},21{{x}^{'}}^{2}+9{{y}^{'}}^{2}+4{x}^{'}-4\sqrt{3}{y}^{'}-6=0\)
Symbols used here
The non-negative number whose square (n-th power) is x.
The usual name for an angle.
Ratios of sides in a right triangle; coordinates on the unit circle.
1/360 of a full turn. 180° = π radians.
The two sides are different.
Distance from zero: |−3| = 3. For a complex number, distance from the origin.
Number of k-element subsets of n things: n!/(k!(n−k)!).
Ratio of a circle's circumference to its diameter, 3.14159…
2.71828…, the base whose exponential is its own derivative.
i² = −1.
The exponent b must be raised to for x; ln uses base e.
A quantity with magnitude and direction; a column of numbers.
How to: Rotation of Axes
- Identify nondegenerate conic sections given their general form equations.
- Use rotation of axes formulas.
- Write equations of rotated conics in standard form.
- Identify conics without rotating axes.
- Using rotation of axes formulas.
- Identify conic sections by their equations. (IA 11.4.3)
- Find
- Substitute the expression for
Questions people ask
What is a function, really?
A rule that assigns exactly one output to each input. The vertical-line test on a graph is the same idea: no input may have two outputs.
Why do we need complex numbers?
Because x² + 1 = 0 has no real solution, and allowing one new number i with i² = −1 makes every polynomial equation solvable. They then turn out to describe rotation, waves and alternating current more naturally than real numbers do.
તમારા પોતાના પ્રયત્ન કરો
Parts of this page are adapted from OpenStax Precalculus 2e (CC BY-NC-SA 4.0). Condensed and re-explained here; errors are ours.
આમાં વધુ Precalculus
Complex numbersPolynomial functionsRational functionsSequences and seriesThe binomial theoremConic sectionsVectorsExponential and logarithmic functionsPolynomial division and the remainder theoremParametric equations and polar coordinates