Solve Equations in Quadratic Form

Solve Equations in Quadratic Form

Sometimes when we factored trinomials, the trinomial did not appear to be in the ax² + bx + c form. So we factored by substitution allowing us to make it fit the ax² + bx + c form. We used the standard \(u\) for the substitution.

To factor the expression x⁴ − 4x² − 5, we noticed the variable part of the middle term is x² and its square, x⁴, is the variable part of the first term. (We know \({({x}^{2})}^{2}={x}^{4}.\)) So we let u = x² and factored.

Let \(u={x}^{2}\) and substitute.
Factor the trinomial.
Replace u with \({x}^{2}\).

Similarly, sometimes an equation is not in the ax² + bx + c = 0 form but looks much like a quadratic equation. Then, we can often make a thoughtful substitution that will allow us to make it fit the ax² + bx + c = 0 form. If we can make it fit the form, we can then use all of our methods to solve quadratic equations.

Notice that in the quadratic equation ax² + bx + c = 0, the middle term has a variable, x, and its square, x², is the variable part of the first term. Look for this relationship as you try to find a substitution.

Again, we will use the standard u to make a substitution that will put the equation in quadratic form. If the substitution gives us an equation of the form ax² + bx + c = 0, we say the original equation was of quadratic form.

The next example shows the steps for solving an equation in quadratic form.

How to Solve Equations in Quadratic Form

Try it.

Solve: \(6{x}^{4}-7{x}^{2}+2=0\)

Solution

We summarize the steps to solve an equation in quadratic form.

Example

Try it.

Solve: \({(x-2)}^{2}+7(x-2)+12=0.\)

Solution

Prepare for the substitution.
Let \(u=x-2\) and substitute.
Solve by factoring.
Replace \(u\) with \(x-2.\)
Solve for \(x.\)
Check:

Condensed: the full section is in OpenStax Intermediate Algebra 2e.

Key Concepts

• How to solve equations in quadratic form.
  1. Identify a substitution that will put the equation in quadratic form.
  2. Rewrite the equation with the substitution to put it in quadratic form.
  3. Solve the quadratic equation for u.
  4. Substitute the original variable back into the results, using the substitution.
  5. Solve for the original variable.
  6. Check the solutions.

Solve Equations in Quadratic Form

Solve Equations in Quadratic Form

In the following exercises, solve.

Try it.

\({x}^{4}-7{x}^{2}+12=0\)

Solution

\(x=\pm \sqrt{3},x=\pm 2\)

Try it.

\({x}^{4}-9{x}^{2}+18=0\)

Try it.

\({x}^{4}-13{x}^{2}-30=0\)

Solution

\(x=\pm \sqrt{15},x=\pm \sqrt{2}i\)

Try it.

\({x}^{4}+5{x}^{2}-36=0\)

Try it.

\(2{x}^{4}-5{x}^{2}+3=0\)

Solution

\(x=\pm 1,x=\frac{\pm \sqrt{6}}{2}\)

Try it.

\(4{x}^{4}-5{x}^{2}+1=0\)

Try it.

\(2{x}^{4}-7{x}^{2}+3=0\)

Solution

\(x=\pm \sqrt{3},x=\pm \frac{\sqrt{2}}{2}\)

Try it.

\(3{x}^{4}-14{x}^{2}+8=0\)

Try it.

\({(x-3)}^{2}-5(x-3)-36=0\)

Solution

\(x=-1,x=12\)

Try it.

\({(x+2)}^{2}-3(x+2)-54=0\)

Try it.

\({(3y+2)}^{2}+(3y+2)-6=0\)

Solution

\(x=-\frac{5}{3},x=0\)

Try it.

\({(5y-1)}^{2}+3(5y-1)-28=0\)

Try it.

\({({x}^{2}+1)}^{2}-5({x}^{2}+1)+4=0\)

Solution

\(x=0,x=\pm \sqrt{3}\)

Try it.

\({({x}^{2}-4)}^{2}-4({x}^{2}-4)+3=0\)

Try it.

\(2{({x}^{2}-5)}^{2}-5({x}^{2}-5)+2=0\)

Solution

\(x=\pm \frac{\sqrt{22}}{2},x=\pm \sqrt{7}\)

Try it.

\(2{({x}^{2}-5)}^{2}-7({x}^{2}-5)+6=0\)

Try it.

\(x-\sqrt{x}-20=0\)

Solution

\(x=25\)

Try it.

\(x-8\sqrt{x}+15=0\)

Try it.

\(x+6\sqrt{x}-16=0\)

Solution

\(x=4\)

Try it.

\(x+4\sqrt{x}-21=0\)

Try it.

\(6x+\sqrt{x}-2=0\)

Solution

\(x=\frac{1}{4}\)

Try it.

\(6x+\sqrt{x}-1=0\)

Try it.

\(10x-17\sqrt{x}+3=0\)

Solution

\(x=\frac{1}{25},x=\frac{9}{4}\)

Try it.

\(12x+5\sqrt{x}-3=0\)

Try it.

\({x}^{\frac{2}{3}}+9{x}^{\frac{1}{3}}+8=0\)

Solution

\(x=-1,x=-512\)

Try it.

\({x}^{\frac{2}{3}}-3{x}^{\frac{1}{3}}=28\)

Try it.

\({x}^{\frac{2}{3}}+4{x}^{\frac{1}{3}}=12\)

Solution

\(x=8,x=-216\)

Try it.

\({x}^{\frac{2}{3}}-11{x}^{\frac{1}{3}}+30=0\)

Try it.

\(6{x}^{\frac{2}{3}}-{x}^{\frac{1}{3}}=12\)

Solution

\(x=\frac{27}{8},x=-\frac{64}{27}\)

Try it.

\(3{x}^{\frac{2}{3}}-10{x}^{\frac{1}{3}}=8\)

Try it.

\(8{x}^{\frac{2}{3}}-43{x}^{\frac{1}{3}}+15=0\)

Solution

\(x=\frac{27}{512},x=125\)

Try it.

\(20{x}^{\frac{2}{3}}-23{x}^{\frac{1}{3}}+6=0\)

Try it.

\(x-8{x}^{\frac{1}{2}}+7=0\)

Solution

\(x=1,x=49\)

Try it.

\(2x-7{x}^{\frac{1}{2}}=15\)

Try it.

\(6{x}^{-2}+13{x}^{-1}+5=0\)

Solution

\(x=-2,x=-\frac{3}{5}\)

Try it.

\(15{x}^{-2}-26{x}^{-1}+8=0\)

Try it.

\(8{x}^{-2}-2{x}^{-1}-3=0\)

Solution

\(x=-2,x=\frac{4}{3}\)

Try it.

\(15{x}^{-2}-4{x}^{-1}-4=0\)

Try it.

Explain how to recognize an equation in quadratic form.

Solution

Answers will vary.

Try it.

Explain the procedure for solving an equation in quadratic form.

ⓐ After completing the exercises, use this checklist to evaluate your mastery of the objectives of this section.

ⓑ On a scale of 1-10, how would you rate your mastery of this section in light of your responses on the checklist? How can you improve this?

Sasa Hakuna kifaa cha kupigia hesabu kinachoweza kufanya hesabu, lakini kifaa hicho kinaweza kutumiwa katika hesabu.

⌨ Aina ✎ Chora mstari wa kwanza

hesabu zako zinatatiza

π Utasuluhisha

Zoea (40)

Jaribu kila moja kwenye karatasi kwanza. kufunua jibu la kuchunguza; zile zilizothibitishwa zaweza kufunguliwa katika kitatua kwa kila hatua.

1. Factor by substitution: \({y}^{4}-{y}^{2}-20.\)

   Fanya kazi kwa undani Dokezo

   Lafunua jibu

   \(\left({y}^{2}+4\right)\left({y}^{2}-5\right)\)

2. Factor by substitution: \({(y-4)}^{2}+8(y-4)+15.\)

   Fanya kazi kwa undani Dokezo

   Lafunua jibu

   \((y-1)(y+1)\)

3. Simplify: ⓐ \({x}^{\frac{1}{2}}\cdot {x}^{\frac{1}{4}}\) ⓑ \({({x}^{\frac{1}{3}})}^{2}\) ⓒ \({({x}^{-1})}^{2}.\)

   Fanya kazi kwa undani Dokezo

   Lafunua jibu

   ⓐ \({x}^{\frac{3}{4}}\); ⓑ \({x}^{\frac{2}{3}}\); ⓒ \({x}^{-2}\)

4. Solve: \(6{x}^{4}-7{x}^{2}+2=0\)

   Fanya kazi kwa undani Dokezo

5. Solve: \({x}^{4}-6{x}^{2}+8=0\).

   Fanya kazi kwa undani Dokezo

   Lafunua jibu

   \(x=\sqrt{2},x=\text{-}\sqrt{2},x=2,x=-2\)

6. Solve: \({x}^{4}-11{x}^{2}+28=0\).

   Fanya kazi kwa undani Dokezo

   Lafunua jibu

   \(x=\sqrt{7},x=\text{-}\sqrt{7},x=2,x=-2\)

7. Solve: \({(x-2)}^{2}+7(x-2)+12=0.\)

   Fanya kazi kwa undani Dokezo

   Lafunua jibu

   Prepare for the substitution.
   Let \(u=x-2\) and substitute.
   Solve by factoring.
   Replace \(u\) with \(x-2.\)
   Solve for \(x.\)
   Check:

8. Solve: \({(x-5)}^{2}+6(x-5)+8=0.\)

   Fanya kazi kwa undani Dokezo

   Lafunua jibu

   \(x=3,x=1\)

9. Solve: \({(y-4)}^{2}+8(y-4)+15=0.\)

   Fanya kazi kwa undani Dokezo

   Lafunua jibu

   \(y=-1,y=1\)

10. Solve: \(x-3\sqrt{x}+2=0.\)

    Fanya kazi kwa undani Dokezo

    Lafunua jibu

    The \(\sqrt{x}\) in the middle term, is squared in the first term \({(\sqrt{x})}^{2}=x.\) If we let \(u=\sqrt{x}\) and substitute, our trinomial will be in ax² + bx + c = 0 form.

    Rewrite the trinomial to prepare for the substitution.
    Let \(u=\sqrt{x}\) and substitute.
    Solve by factoring.
    Replace u with \(\sqrt{x}.\)
    Solve for x, by squaring both sides.
    Check:

11. Solve: \(x-7\sqrt{x}+12=0.\)

    Fanya kazi kwa undani Dokezo

    Lafunua jibu

    \(x=9,x=16\)

12. Solve: \(x-6\sqrt{x}+8=0.\)

    Fanya kazi kwa undani Dokezo

    Lafunua jibu

    \(x=4,x=16\)

13. Solve: \({x}^{\frac{2}{3}}-2{x}^{\frac{1}{3}}-24=0.\)

    Fanya kazi kwa undani Dokezo

    Lafunua jibu

    The \({x}^{\frac{1}{3}}\) in the middle term is squared in the first term \({({x}^{\frac{1}{3}})}^{2}={x}^{\frac{2}{3}}.\) If we let \(u={x}^{\frac{1}{3}}\) and substitute, our trinomial will be in ax² + bx + c = 0 form.

    Rewrite the trinomial to prepare for the substitution.
    Let \(u={x}^{\frac{1}{3}}\) and substitute.
    Solve by factoring.
    Replace u with \({x}^{\frac{1}{3}}.\)
    Solve for \(x\) by cubing both sides.
    Check:

14. Solve: \({x}^{\frac{2}{3}}-5{x}^{\frac{1}{3}}-14=0.\)

    Fanya kazi kwa undani Dokezo

    Lafunua jibu

    \(x=-8,x=343\)

15. Solve: \({x}^{\frac{1}{2}}-8{x}^{\frac{1}{4}}+15=0.\)

    Fanya kazi kwa undani Dokezo

    Lafunua jibu

    \(x=81,x=625\)

16. Solve: \(3{x}^{-2}-7{x}^{-1}+2=0.\)

    Fanya kazi kwa undani Dokezo

    Lafunua jibu

    The \({x}^{-1}\) in the middle term is squared in the first term \({({x}^{-1})}^{2}={x}^{-2}.\) If we let \(u={x}^{-1}\) and substitute, our trinomial will be in ax² + bx + c = 0 form.

    Rewrite the trinomial to prepare for the substitution.
    Let \(u={x}^{-1}\) and substitute.
    Solve by factoring.
    Replace u with \({x}^{-1}.\)
    Solve for \(x\) by taking the reciprocal since \({x}^{-1}=\frac{1}{x}.\)
    Check:

17. Solve: \(8{x}^{-2}-10{x}^{-1}+3=0.\)

    Fanya kazi kwa undani Dokezo

    Lafunua jibu

    \(x=\frac{4}{3}x=2\)

18. Solve: \(6{x}^{-2}-23{x}^{-1}+20=0.\)

    Fanya kazi kwa undani Dokezo

    Lafunua jibu

    \(x=\frac{2}{5},x=\frac{3}{4}\)

19. \({x}^{4}-7{x}^{2}+12=0\)

    Fanya kazi kwa undani Dokezo

    Lafunua jibu

    \(x=\pm \sqrt{3},x=\pm 2\)

20. \({x}^{4}-9{x}^{2}+18=0\)

    Fanya kazi kwa undani Dokezo

21. \({x}^{4}-13{x}^{2}-30=0\)

    Fanya kazi kwa undani Dokezo

    Lafunua jibu

    \(x=\pm \sqrt{15},x=\pm \sqrt{2}i\)

22. \({x}^{4}+5{x}^{2}-36=0\)

    Fanya kazi kwa undani Dokezo

23. \(2{x}^{4}-5{x}^{2}+3=0\)

    Fanya kazi kwa undani Dokezo

    Lafunua jibu

    \(x=\pm 1,x=\frac{\pm \sqrt{6}}{2}\)

24. \(4{x}^{4}-5{x}^{2}+1=0\)

    Fanya kazi kwa undani Dokezo

25. \(2{x}^{4}-7{x}^{2}+3=0\)

    Fanya kazi kwa undani Dokezo

    Lafunua jibu

    \(x=\pm \sqrt{3},x=\pm \frac{\sqrt{2}}{2}\)

26. \(3{x}^{4}-14{x}^{2}+8=0\)

    Fanya kazi kwa undani Dokezo

27. \({(x-3)}^{2}-5(x-3)-36=0\)

    Fanya kazi kwa undani Dokezo

    Lafunua jibu

    \(x=-1,x=12\)

28. \({(x+2)}^{2}-3(x+2)-54=0\)

    Fanya kazi kwa undani Dokezo

29. \({(3y+2)}^{2}+(3y+2)-6=0\)

    Fanya kazi kwa undani Dokezo

    Lafunua jibu

    \(x=-\frac{5}{3},x=0\)

30. \({(5y-1)}^{2}+3(5y-1)-28=0\)

    Fanya kazi kwa undani Dokezo

31. \({({x}^{2}+1)}^{2}-5({x}^{2}+1)+4=0\)

    Fanya kazi kwa undani Dokezo

    Lafunua jibu

    \(x=0,x=\pm \sqrt{3}\)

32. \({({x}^{2}-4)}^{2}-4({x}^{2}-4)+3=0\)

    Fanya kazi kwa undani Dokezo

33. \(2{({x}^{2}-5)}^{2}-5({x}^{2}-5)+2=0\)

    Fanya kazi kwa undani Dokezo

    Lafunua jibu

    \(x=\pm \frac{\sqrt{22}}{2},x=\pm \sqrt{7}\)

34. \(2{({x}^{2}-5)}^{2}-7({x}^{2}-5)+6=0\)

    Fanya kazi kwa undani Dokezo

35. \(x-\sqrt{x}-20=0\)

    Fanya kazi kwa undani Dokezo

    Lafunua jibu

    \(x=25\)

36. \(x-8\sqrt{x}+15=0\)

    Fanya kazi kwa undani Dokezo

37. \(x+6\sqrt{x}-16=0\)

    Fanya kazi kwa undani Dokezo

    Lafunua jibu

    \(x=4\)

38. \(x+4\sqrt{x}-21=0\)

    Fanya kazi kwa undani Dokezo

39. \(6x+\sqrt{x}-2=0\)

    Fanya kazi kwa undani Dokezo

    Lafunua jibu

    \(x=\frac{1}{4}\)

40. \(6x+\sqrt{x}-1=0\)

    Fanya kazi kwa undani Dokezo

Keep your own working

Akaunti la bure laongezea mambo makuu juu ya kila somo, rekodi ya yale ambayo umemaliza, matatizo yako ya kutatua katika sehemu moja, na mtunzaji unayeweza kuuliza juu ya ukurasa huu.

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\sqrt{x}square root, n-th root
The non-negative number whose square (n-th power) is x.iimaginary unit
i² = −1.\pmplus or minus
Both signs at once: x = 3 ± 2 means 5 and 1.\neqnot equal
The two sides are different.\leqless/greater than or equal
Inequalities that allow equality; < and > exclude it.|x|absolute value / modulus
Distance from zero: |−3| = 3. For a complex number, distance from the origin.\log_b xlogarithm, natural log
The exponent b must be raised to for x; ln uses base e.\mathbb{N}number sets
Naturals, integers, rationals, reals, complex numbers.

Jinsi ya Kufaulu: Solve Equations in Quadratic Form

1. Solve equations in quadratic form
2. Identify a substitution that will put the equation in quadratic form.
3. Rewrite the equation with the substitution to put it in quadratic form.
4. Solve the quadratic equation for
5. Substitute the original variable back into the results, using the substitution.
6. Solve for the original variable.
7. Check the solutions.
8. How to solve equations in quadratic form.

Maswali ambayo watu huuliza

What does it mean to solve an equation?

To find every value of the unknown that makes both sides equal. Each step is an operation applied to both sides that keeps the solution set the same, until the unknown stands alone.

Why do I sometimes get two answers?

A quadratic can cross the axis twice, so it can have two solutions. A degree-n polynomial has up to n. The graph shows where each one comes from.

How do I know whether to factor or use the quadratic formula?

Try factoring for a few seconds: look for two numbers that multiply to a·c and add to b. If nothing obvious appears, the discriminant b² − 4ac tells you how many real roots there are, and the formula finds them without guessing.

Sehemu za ukurasa huu zimebadilishwa kutoka kwa OpenStax Intermediate Algebra 2e (CC BY-NC-SA 4.0). Tumekamatwa na kukosolewa tena hapa; makosa ni yetu. Kwa sababu chanzo chake kimeidhinishwa kwa matumizi yasiyo ya kibiashara, somo hili ni bure, haliuziwi chochote, na hushirikiwa na leseni ileile.

Mengi zaidi katika Algebra

Linear equationsQuadratic equationsSystems of equationsInequalitiesFactoringExpandingSimplifying expressionsFunctions and graphsExponential and logarithmic equationsPolynomial equationsAbsolute value