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Add and Subtract Rational Expressions with a Common Denominator
Add rational expressions with a common denominator
Add Rational Expressions with a Common Denominator
What is the first step you take when you add numerical fractions? You check if they have a common denominator. If they do, you add the numerators and place the sum over the common denominator. If they do not have a common denominator, you find one before you add.
It is the same with rational expressions. To add rational expressions, they must have a common denominator. When the denominators are the same, you add the numerators and place the sum over the common denominator.
We will add two numerical fractions first, to remind us of how this is done.
Example
Try it.
Add: \(\frac{5}{18}+\frac{7}{18}.\)
Solution
| \(\frac{5}{18}+\frac{7}{18}\) | |
| The fractions have a common denominator, so add the numerators and place the sum over the common denominator. | \(\frac{5+7}{18}\) |
| Add in the numerator. | \(\frac{12}{18}\) |
| Factor the numerator and denominator to show the common factors. | \(\frac{6\cdot 2}{6\cdot 3}\) |
| Remove common factors. | \(\frac{6\cdot 2}{6\cdot 3}\) |
| Simplify. | \(\frac{2}{3}\) |
Remember, we do not allow values that would make the denominator zero. What value of \(y\) should be excluded in the next example?
Example
Try it.
Add: \(\frac{3y}{4y-3}+\frac{7}{4y-3}.\)
Solution
| \(\frac{3y}{4y-3}+\frac{7}{4y-3}\) | |
| The fractions have a common denominator, so add the numerators and place the sum over the common denominator. | \(\frac{3y+7}{4y-3}\) |
The numerator and denominator cannot be factored. The fraction is simplified.
Example
Try it.
Add: \(\frac{7x+12}{x+3}+\frac{{x}^{2}}{x+3}.\)
Solution
| \(\frac{7x+12}{x+3}+\frac{{x}^{2}}{x+3}\) | |
| The fractions have a common denominator, so add the numerators and place the sum over the common denominator. | \(\frac{7x+12+{x}^{2}}{x+3}\) |
| Write the degrees in descending order. | \(\frac{{x}^{2}+7x+12}{x+3}\) |
| Factor the numerator. | \(\frac{(x+3)(x+4)}{x+3}\) |
| Simplify by removing common factors. | \(\frac{(x+3)(x+4)}{x+3}\) |
| Simplify. | \(x+4\) |
Subtract Rational Expressions with a Common Denominator
To subtract rational expressions, they must also have a common denominator. When the denominators are the same, you subtract the numerators and place the difference over the common denominator.
We always simplify rational expressions. Be sure to factor, if possible, after you subtract the numerators so you can identify any common factors.
Example
Try it.
Subtract: \(\frac{{n}^{2}}{n-10}-\frac{100}{n-10}.\)
Solution
| \(\frac{{n}^{2}}{n-10}-\frac{100}{n-10}\) | |
| The fractions have a common denominator, so subtract the numerators and place the difference over the common denominator. | \(\frac{{n}^{2}-100}{n-10}\) |
| Factor the numerator. | \(\frac{(n-10)(n+10)}{n-10}\) |
| Simplify by removing common factors. | \(\frac{(n-10)(n+10)}{n-10}\) |
| Simplify. | \(n+10\) |
Be careful of the signs when you subtract a binomial!
Example
Try it.
Subtract: \(\frac{{y}^{2}}{y-6}-\frac{2y+24}{y-6}.\)
Solution
| \(\frac{{y}^{2}}{y-6}-\frac{2y+24}{y-6}\) | |
| The fractions have a common denominator, so subtract the numerators and place the difference over the common denominator. | \(\frac{{y}^{2}-(2y+24)}{y-6}\) |
| Distribute the sign in the numerator. | \(\frac{{y}^{2}-2y-24}{y-6}\) |
| Factor the numerator. | \(\frac{(y-6)(y+4)}{y-6}\) |
| Remove common factors. | \(\frac{(y-6)(y+4)}{y-6}\) |
| Simplify. | \(y+4\) |
Example
Try it.
Subtract: \(\frac{5{x}^{2}-7x+3}{{x}^{2}-3x-18}-\frac{4{x}^{2}+x-9}{{x}^{2}-3x-18}.\)
Solution
| \(\frac{5{x}^{2}-7x+3}{{x}^{2}-3x-18}-\frac{4{x}^{2}+x-9}{{x}^{2}-3x-18}\) | |
| Subtract the numerators and place the difference over the common denominator. | \(\frac{5{x}^{2}-7x+3-(4{x}^{2}+x-9)}{{x}^{2}-3x-18}\) |
| Distribute the sign in the numerator. | \(\frac{5{x}^{2}-7x+3-4{x}^{2}-x+9}{{x}^{2}-3x-18}\) |
| Combine like terms. | \(\frac{{x}^{2}-8x+12}{{x}^{2}-3x-18}\) |
| Factor the numerator and the denominator. | \(\frac{(x-2)(x-6)}{(x+3)(x-6)}\) |
| Simplify by removing common factors. | \(\frac{(x-2)(x-6)}{(x+3)(x-6)}\) |
| Simplify. | \(\frac{(x-2)}{(x+3)}\) |
Add and Subtract Rational Expressions whose Denominators are Opposites
When the denominators of two rational expressions are opposites, it is easy to get a common denominator. We just have to multiply one of the fractions by \(\frac{-1}{-1}\).
Let’s see how this works.
| Multiply the second fraction by \(\frac{-1}{-1}\). | |
| The denominators are the same. | |
| Simplify. |
Example
Try it.
Add: \(\frac{4u-1}{3u-1}+\frac{u}{1-3u}.\)
Solution
| The denominators are opposites, so multiply the second fraction by \(\frac{-1}{-1}\). | |
| Simplify the second fraction. | |
| The denominators are the same. Add the numerators. | |
| Simplify. | |
| Simplify. |
Example
Try it.
Subtract: \(\frac{{m}^{2}-6m}{{m}^{2}-1}-\frac{3m+2}{1-{m}^{2}}.\)
Solution
| The denominators are opposites, so multiply the second fraction by \(\frac{-1}{-1}\). | |
| Simplify the second fraction. | |
| The denominators are the same. Subtract the numerators. | |
| Distribute. | \(\frac{{m}^{2}-6m+3m+2}{{m}^{2}-1}\) |
| Combine like terms. | |
| Factor the numerator and denominator. | |
| Simplify by removing common factors. | |
| Simplify. |
Key Concepts
- Rational Expression Addition
- If \(p,q,\ \text{and}\ r\) are polynomials where \(r\ne 0\), then
\[\frac{p}{r}+\frac{q}{r}=\frac{p+q}{r}\] - To add rational expressions with a common denominator, add the numerators and place the sum over the common denominator.
- If \(p,q,\ \text{and}\ r\) are polynomials where \(r\ne 0\), then
- Rational Expression Subtraction
- If \(p,q,\ \text{and}\ r\) are polynomials where \(r\ne 0\), then
\[\frac{p}{r}-\frac{q}{r}=\frac{p-q}{r}\] - To subtract rational expressions, subtract the numerators and place the difference over the common denominator.
- If \(p,q,\ \text{and}\ r\) are polynomials where \(r\ne 0\), then
Add and Subtract Rational Expressions with a Common Denominator
Add Rational Expressions with a Common Denominator
In the following exercises, add.
Try it.
\(\frac{2}{15}+\frac{7}{15}\)
Solution
\(\frac{3}{5}\)
Try it.
\(\frac{4}{21}+\frac{3}{21}\)
Try it.
\(\frac{7}{24}+\frac{11}{24}\)
Solution
\(\frac{3}{4}\)
Try it.
\(\frac{7}{36}+\frac{13}{36}\)
Try it.
\(\frac{3a}{a-b}+\frac{1}{a-b}\)
Solution
\(\frac{3a+1}{a-b}\)
Try it.
\(\frac{3c}{4c-5}+\frac{5}{4c-5}\)
Try it.
\(\frac{d}{d+8}+\frac{5}{d+8}\)
Solution
\(\frac{d+5}{d+8}\)
Try it.
\(\frac{7m}{2m+n}+\frac{4}{2m+n}\)
Try it.
\(\frac{{p}^{2}+10p}{p+2}+\frac{16}{p+2}\)
Solution
\(p+8\)
Try it.
\(\frac{{q}^{2}+12q}{q+3}+\frac{27}{q+3}\)
Try it.
\(\frac{2{r}^{2}}{2r-1}+\frac{15r-8}{2r-1}\)
Solution
\(r+8\)
Try it.
\(\frac{3{s}^{2}}{3s-2}+\frac{13s-10}{3s-2}\)
Try it.
\(\frac{8{t}^{2}}{t+4}+\frac{32t}{t+4}\)
Solution
\(8t\)
Try it.
\(\frac{6{v}^{2}}{v+5}+\frac{30v}{v+5}\)
Try it.
\(\frac{2{w}^{2}}{{w}^{2}-16}+\frac{8w}{{w}^{2}-16}\)
Solution
\(\frac{2w}{w-4}\)
Try it.
\(\frac{7{x}^{2}}{{x}^{2}-9}+\frac{21x}{{x}^{2}-9}\)
Subtract Rational Expressions with a Common Denominator
In the following exercises, subtract.
Try it.
\(\frac{{y}^{2}}{y+8}-\frac{64}{y+8}\)
Solution
\(y-8\)
Try it.
\(\frac{{z}^{2}}{z+2}-\frac{4}{z+2}\)
Try it.
\(\frac{9{a}^{2}}{3a-7}-\frac{49}{3a-7}\)
Solution
\(3a+7\)
Try it.
\(\frac{25{b}^{2}}{5b-6}-\frac{36}{5b-6}\)
Try it.
\(\frac{{c}^{2}}{c-8}-\frac{6c+16}{c-8}\)
Solution
\(c+2\)
Try it.
\(\frac{{d}^{2}}{d-9}-\frac{6d+27}{d-9}\)
Try it.
\(\frac{3{m}^{2}}{6m-30}-\frac{21m-30}{6m-30}\)
Solution
\(\frac{m-2}{2}\)
Try it.
\(\frac{2{n}^{2}}{4n-32}-\frac{18n-16}{4n-32}\)
Try it.
\(\frac{6{p}^{2}+3p+4}{{p}^{2}+4p-5}-\frac{5{p}^{2}+p+7}{{p}^{2}+4p-5}\)
Solution
\(\frac{p+3}{p+5}\)
Try it.
\(\frac{5{q}^{2}+3q-9}{{q}^{2}+6q+8}-\frac{4{q}^{2}+9q+7}{{q}^{2}+6q+8}\)
Try it.
\(\frac{5{r}^{2}+7r-33}{{r}^{2}-49}-\frac{4{r}^{2}+5r+30}{{r}^{2}-49}\)
Solution
\(\frac{r+9}{r+7}\)
Try it.
\(\frac{7{t}^{2}-t-4}{{t}^{2}-25}-\frac{6{t}^{2}+12t-44}{{t}^{2}-25}\)
Add and Subtract Rational Expressions whose Denominators are Opposites
In the following exercises, add.
Try it.
\(\frac{10v}{2v-1}+\frac{2v+4}{1-2v}\)
Solution
\(4\)
Try it.
\(\frac{20w}{5w-2}+\frac{5w+6}{2-5w}\)
Try it.
\(\frac{10{x}^{2}+16x-7}{8x-3}+\frac{2{x}^{2}+3x-1}{3-8x}\)
Solution
\(x+2\)
Try it.
\(\frac{6{y}^{2}+2y-11}{3y-7}+\frac{3{y}^{2}-3y+17}{7-3y}\)
In the following exercises, subtract.
Try it.
\(\frac{{z}^{2}+6z}{{z}^{2}-25}-\frac{3z+20}{25-{z}^{2}}\)
Solution
\(\frac{z+4}{z-5}\)
Try it.
\(\frac{{a}^{2}+3a}{{a}^{2}-9}-\frac{3a-27}{9-{a}^{2}}\)
Try it.
\(\frac{2{b}^{2}+30b-13}{{b}^{2}-49}-\frac{2{b}^{2}-5b-8}{49-{b}^{2}}\)
Solution
\(\frac{4b-3}{b-7}\)
Try it.
\(\frac{{c}^{2}+5c-10}{{c}^{2}-16}-\frac{{c}^{2}-8c-10}{16-{c}^{2}}\)
Try it.
Donald thinks that \(\frac{3}{x}+\frac{4}{x}\) is \(\frac{7}{2x}.\) Is Donald correct? Explain.
Try it.
Explain how you find the Least Common Denominator of \({x}^{2}+5x+4\) and \({x}^{2}-16.\)
Condensed: the full section is in OpenStax Elementary Algebra 2e.
现在轮到你 没有计算器能解决这一个问题, 但其中的碎片是可以计算的。 请在下面尝试一个, 或者输入您自己的 。
实践 (40)
先在纸上试一个。 将答案显示为检查; 校验的答案可以在求解器中打开, 每一步都可用 。
-
Add: \(\frac{y}{3}+\frac{9}{3}.\)
发送答案
\(\frac{y+9}{3}\)
-
Subtract: \(\frac{10}{x}-\frac{2}{x}.\)
发送答案
\(\frac{8}{x}\)
-
Factor completely: \(8{n}^{5}-20{n}^{3}.\)
-
Factor completely: \(45{a}^{3}-5a{b}^{2}.\)
发送答案
\(5a\left(3a-b\right)\left(3a+b\right)\)
-
Add: \(\frac{5}{18}+\frac{7}{18}.\)
发送答案
\(\frac{5}{18}+\frac{7}{18}\) The fractions have a common
denominator, so add the numerators and
place the sum over the common denominator.\(\frac{5+7}{18}\) Add in the numerator. \(\frac{12}{18}\) Factor the numerator and denominator to
show the common factors.\(\frac{6\cdot 2}{6\cdot 3}\) Remove common factors. \(\frac{6\cdot 2}{6\cdot 3}\) Simplify. \(\frac{2}{3}\) -
Add: \(\frac{7}{16}+\frac{5}{16}.\)
发送答案
\(\frac{3}{4}\)
-
Add: \(\frac{3}{10}+\frac{1}{10}.\)
发送答案
\(\frac{2}{5}\)
-
Add: \(\frac{3y}{4y-3}+\frac{7}{4y-3}.\)
发送答案
\(\frac{3y}{4y-3}+\frac{7}{4y-3}\) The fractions have a common
denominator, so add the numerators and
place the sum over the common denominator.\(\frac{3y+7}{4y-3}\) The numerator and denominator cannot be factored. The fraction is simplified.
-
Add: \(\frac{5x}{2x+3}+\frac{2}{2x+3}.\)
发送答案
\(\frac{5x+2}{2x+3}.\)
-
Add: \(\frac{x}{x-2}+\frac{1}{x-2}.\)
发送答案
\(\frac{x+1}{x-2}\)
-
Add: \(\frac{7x+12}{x+3}+\frac{{x}^{2}}{x+3}.\)
发送答案
\(\frac{7x+12}{x+3}+\frac{{x}^{2}}{x+3}\) The fractions have a common
denominator, so add the numerators and
place the sum over the common denominator.\(\frac{7x+12+{x}^{2}}{x+3}\) Write the degrees in descending order. \(\frac{{x}^{2}+7x+12}{x+3}\) Factor the numerator. \(\frac{(x+3)(x+4)}{x+3}\) Simplify by removing common factors. \(\frac{(x+3)(x+4)}{x+3}\) Simplify. \(x+4\) -
Add: \(\frac{9x+14}{x+7}+\frac{{x}^{2}}{x+7}.\)
发送答案
\(x+2\)
-
Add: \(\frac{{x}^{2}+8x}{x+5}+\frac{15}{x+5}.\)
发送答案
\(x+3\)
-
Subtract: \(\frac{{n}^{2}}{n-10}-\frac{100}{n-10}.\)
发送答案
\(\frac{{n}^{2}}{n-10}-\frac{100}{n-10}\) The fractions have a common
denominator, so subtract the numerators
and place the difference over the common denominator.\(\frac{{n}^{2}-100}{n-10}\) Factor the numerator. \(\frac{(n-10)(n+10)}{n-10}\) Simplify by removing common factors. \(\frac{(n-10)(n+10)}{n-10}\) Simplify. \(n+10\) -
Subtract: \(\frac{{x}^{2}}{x+3}-\frac{9}{x+3}.\)
发送答案
\(x-3\)
-
Subtract: \(\frac{4{x}^{2}}{2x-5}-\frac{25}{2x-5}.\)
发送答案
\(2x+5\)
-
Subtract: \(\frac{{y}^{2}}{y-6}-\frac{2y+24}{y-6}.\)
发送答案
\(\frac{{y}^{2}}{y-6}-\frac{2y+24}{y-6}\) The fractions have a common
denominator, so subtract the numerators
and place the difference over the common denominator.\(\frac{{y}^{2}-(2y+24)}{y-6}\) Distribute the sign in the numerator. \(\frac{{y}^{2}-2y-24}{y-6}\) Factor the numerator. \(\frac{(y-6)(y+4)}{y-6}\) Remove common factors. \(\frac{(y-6)(y+4)}{y-6}\) Simplify. \(y+4\) -
Subtract: \(\frac{{n}^{2}}{n-4}-\frac{n+12}{n-4}.\)
发送答案
\(n+3\)
-
Subtract: \(\frac{{y}^{2}}{y-1}-\frac{9y-8}{y-1}.\)
发送答案
\(y-8\)
-
Subtract: \(\frac{5{x}^{2}-7x+3}{{x}^{2}-3x-18}-\frac{4{x}^{2}+x-9}{{x}^{2}-3x-18}.\)
发送答案
\(\frac{5{x}^{2}-7x+3}{{x}^{2}-3x-18}-\frac{4{x}^{2}+x-9}{{x}^{2}-3x-18}\) Subtract the numerators and place the
difference over the common denominator.\(\frac{5{x}^{2}-7x+3-(4{x}^{2}+x-9)}{{x}^{2}-3x-18}\) Distribute the sign in the numerator. \(\frac{5{x}^{2}-7x+3-4{x}^{2}-x+9}{{x}^{2}-3x-18}\) Combine like terms. \(\frac{{x}^{2}-8x+12}{{x}^{2}-3x-18}\) Factor the numerator and the denominator. \(\frac{(x-2)(x-6)}{(x+3)(x-6)}\) Simplify by removing common factors. \(\frac{(x-2)(x-6)}{(x+3)(x-6)}\) Simplify. \(\frac{(x-2)}{(x+3)}\) -
Subtract: \(\frac{4{x}^{2}-11x+8}{{x}^{2}-3x+2}-\frac{3{x}^{2}+x-3}{{x}^{2}-3x+2}.\)
发送答案
\(\frac{x-11}{x-2}\)
-
Subtract: \(\frac{6{x}^{2}-x+20}{{x}^{2}-81}-\frac{5{x}^{2}+11x-7}{{x}^{2}-81}.\)
发送答案
\(\frac{x-3}{x+9}\)
-
Add: \(\frac{4u-1}{3u-1}+\frac{u}{1-3u}.\)
发送答案
The denominators are opposites, so multiply the second fraction by \(\frac{-1}{-1}\). Simplify the second fraction. The denominators are the same. Add the numerators. Simplify. Simplify. -
Add: \(\frac{8x-15}{2x-5}+\frac{2x}{5-2x}.\)
发送答案
\(3\)
-
Add: \(\frac{6{y}^{2}+7y-10}{4y-7}+\frac{2{y}^{2}+2y+11}{7-4y}.\)
发送答案
\(y+3\)
-
Subtract: \(\frac{{m}^{2}-6m}{{m}^{2}-1}-\frac{3m+2}{1-{m}^{2}}.\)
发送答案
The denominators are opposites, so multiply the second fraction by \(\frac{-1}{-1}\). Simplify the second fraction. The denominators are the same. Subtract the numerators. Distribute. \(\frac{{m}^{2}-6m+3m+2}{{m}^{2}-1}\) Combine like terms. Factor the numerator and denominator. Simplify by removing common factors. Simplify. -
Subtract: \(\frac{{y}^{2}-5y}{{y}^{2}-4}-\frac{6y-6}{4-{y}^{2}}.\)
发送答案
\(\frac{y+3}{y+2}\)
-
Subtract: \(\frac{2{n}^{2}+8n-1}{{n}^{2}-1}-\frac{{n}^{2}-7n-1}{1-{n}^{2}}.\)
发送答案
\(\frac{3n-2}{n-1}\)
-
\(\frac{2}{15}+\frac{7}{15}\)
发送答案
\(\frac{3}{5}\)
-
\(\frac{4}{21}+\frac{3}{21}\)
-
\(\frac{7}{24}+\frac{11}{24}\)
发送答案
\(\frac{3}{4}\)
-
\(\frac{7}{36}+\frac{13}{36}\)
-
\(\frac{3a}{a-b}+\frac{1}{a-b}\)
发送答案
\(\frac{3a+1}{a-b}\)
-
\(\frac{3c}{4c-5}+\frac{5}{4c-5}\)
-
\(\frac{d}{d+8}+\frac{5}{d+8}\)
发送答案
\(\frac{d+5}{d+8}\)
-
\(\frac{7m}{2m+n}+\frac{4}{2m+n}\)
-
\(\frac{{p}^{2}+10p}{p+2}+\frac{16}{p+2}\)
发送答案
\(p+8\)
-
\(\frac{{q}^{2}+12q}{q+3}+\frac{27}{q+3}\)
-
\(\frac{2{r}^{2}}{2r-1}+\frac{15r-8}{2r-1}\)
发送答案
\(r+8\)
-
\(\frac{3{s}^{2}}{3s-2}+\frac{13s-10}{3s-2}\)
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如何: Add and Subtract Rational Expressions with a Common Denominator
- Add rational expressions with a common denominator
- Subtract rational expressions with a common denominator
- Add and subtract rational expressions whose denominators are opposites
- If
- To add rational expressions with a common denominator, add the numerators and place the sum over the common denominator.
- If
- To subtract rational expressions, subtract the numerators and place the difference over the common denominator.
人们对人们提出的问题
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.
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