Add and Subtract Rational Expressions with Unlike Denominators

Find the Least Common Denominator of Rational Expressions

When we add or subtract rational expressions with unlike denominators we will need to get common denominators. If we review the procedure we used with numerical fractions, we will know what to do with rational expressions.

Let’s look at the example \(\frac{7}{12}+\frac{5}{18}\) from Foundations. Since the denominators are not the same, the first step was to find the least common denominator (LCD). Remember, the LCD is the least common multiple of the denominators. It is the smallest number we can use as a common denominator.

To find the LCD of 12 and 18, we factored each number into primes, lining up any common primes in columns. Then we “brought down” one prime from each column. Finally, we multiplied the factors to find the LCD.

\[\frac{\begin{array}{l} \\ \\ 12=2\cdot 2\cdot 3 \\ 18=2\cdot 3\cdot 3\end{array}}{\begin{array}{l} \\ \\ \text{LCD}=2\cdot 2\cdot 3\cdot 3 \\ \text{LCD}=36\end{array}}\]

We do the same thing for rational expressions. However, we leave the LCD in factored form.

Example

Try it.

Find the LCD for \(\frac{8}{{x}^{2}-2x-3},\frac{3x}{{x}^{2}+4x+3}\).

Solution

	\(\text{Find the LCD for}\ \frac{8}{{x}^{2}-2x-3},\frac{3x}{{x}^{2}+4x+3}.\)
Factor each expression completely, lining up common factors. Bring down the columns.	\(\frac{\begin{array}{l}{x}^{2}-2x-3=(x+1)(x-3) \\ {x}^{2}+4x+3=(x+1)\ (x+3)\end{array}}{\ \text{LCD}=(x+1)(x-3)(x+3)}\)
Multiply the factors.	\(\text{The LCD is}\ (x+1)(x-3)(x+3).\)

Find Equivalent Rational Expressions

When we add numerical fractions, once we find the LCD, we rewrite each fraction as an equivalent fraction with the LCD.

We will do the same thing for rational expressions.

Example

Try it.

Rewrite as equivalent rational expressions with denominator \((x+1)(x-3)(x+3)\): \(\frac{8}{{x}^{2}-2x-3},\frac{3x}{{x}^{2}+4x+3}.\)

Solution

Factor each denominator.
Find the LCD.
Multiply each denominator by the 'missing' factor and multiply each numerator by the same factor.
Simplify the numerators.

Add Rational Expressions with Different Denominators

Now we have all the steps we need to add rational expressions with different denominators. As we have done previously, we will do one example of adding numerical fractions first.

Example

Try it.

Add: \(\frac{7}{12}+\frac{5}{18}.\)

Solution

Find the LCD of 12 and 18.
Rewrite each fraction as an equivalent fraction with the LCD.
Add the fractions.
The fraction cannot be simplified.

Now we will add rational expressions whose denominators are monomials.

Example

Try it.

Add: \(\frac{5}{12{x}^{2}y}+\frac{4}{21x{y}^{2}}.\)

Solution

Find the LCD of \(12{x}^{2}y\) and \(21x{y}^{2}\).
Rewrite each rational expression as an equivalent fraction with the LCD.
Simplify.
Add the rational expressions.
There are no factors common to the numerator and denominator. The fraction cannot be simplified.

Now we are ready to tackle polynomial denominators.

How to Add Rational Expressions with Different Denominators

Try it.

Add: \(\frac{3}{x-3}+\frac{2}{x-2}.\)

Solution

The steps to use to add rational expressions are summarized in the following procedure box.

Example

Try it.

Add: \(\frac{2a}{2ab+{b}^{2}}+\frac{3a}{4{a}^{2}-{b}^{2}}.\)

Solution

Do the expressions have a common denominator? No. Rewrite each expression with the LCD.
Find the LCD.
Rewrite each rational expression as an equivalent rational expression with the LCD.
Simplify the numerators.
Add the rational expressions.
Simplify the numerator.
Factor the numerator.
There are no factors common to the numerator and denominator. The fraction cannot be simplified.

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

Subtract Rational Expressions with Different Denominators

The process we use to subtract rational expressions with different denominators is the same as for addition. We just have to be very careful of the signs when subtracting the numerators.

How to Subtract Rational Expressions with Different Denominators

Try it.

Subtract: \(\frac{x}{x-3}-\frac{x-2}{x+3}.\)

Solution

The steps to take to subtract rational expressions are listed below.

Example

Try it.

Subtract: \(\frac{8y}{{y}^{2}-16}-\frac{4}{y-4}.\)

Solution

Do the expressions have a common denominator? No. Rewrite each expression with the LCD.
Find the LCD.
Rewrite each rational expression as an equivalent rational expression with the LCD.
Simplify the numerators.
Subtract the rational expressions.
Simplify the numerators.
Factor the numerator to look for common factors.
Remove common factors.
Simplify.

There are lots of negative signs in the next example. Be extra careful!

Example

Try it.

Subtract: \(\frac{-3n-9}{{n}^{2}+n-6}-\frac{n+3}{2-n}.\)

Solution

Factor the denominator.
Since \(n-2\) and \(2-n\) are opposites, we will mutliply the second rational expression by\(\frac{-1}{-1}\).
Simplify.
Do the expressions have a common denominator? No.
Find the LCD.
Rewrite each rational expression as an equivalent rational expression with the LCD.
Simplify the numerators.
Simplify the rational expressions.
Simplify the numerator.
Factor the numerator to look for common factors.
Simplify.

When one expression is not in fraction form, we can write it as a fraction with denominator 1.

We follow the same steps as before to find the LCD when we have more than two rational expressions. In the next example we will start by factoring all three denominators to find their LCD.

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

Key Concepts

• Find the Least Common Denominator of Rational Expressions
  1. Factor each expression completely.
  2. List the factors of each expression. Match factors vertically when possible.
  3. Bring down the columns.
  4. Multiply the factors.
• Add or Subtract Rational Expressions
  1. Determine if the expressions have a common denominator.
     Yes, go to step 2.
     No, Rewrite each rational expression with the LCD.
     • Find the LCD.
     • Rewrite each rational expression as an equivalent rational expression with the LCD.
  2. Add or subtract the rational expressions.
  3. Simplify, if possible.

Add and Subtract Rational Expressions with Unlike Denominators

In the following exercises, find the LCD.

Try it.

\(\frac{5}{{x}^{2}-2x-8},\frac{2x}{{x}^{2}-x-12}\)

Solution

\((x-4)(x+2)(x+3)\)

Try it.

\(\frac{8}{{y}^{2}+12y+35},\frac{3y}{{y}^{2}+y-42}\)

Try it.

\(\frac{9}{{z}^{2}+2z-8},\frac{4z}{{z}^{2}-4}\)

Solution

\((z-2)(z+4)(z+2)\)

Try it.

\(\frac{6}{{a}^{2}+14a+45},\frac{5a}{{a}^{2}-81}\)

Try it.

\(\frac{4}{{b}^{2}+6b+9},\frac{2b}{{b}^{2}-2b-15}\)

Solution

\((b+3)(b+3)(b-5)\)

Try it.

\(\frac{5}{{c}^{2}-4c+4},\frac{3c}{{c}^{2}-10c+16}\)

Try it.

\(\frac{2}{3{d}^{2}+14d-5},\frac{5d}{3{d}^{2}-19d+6}\)

Solution

\((3d-1)(d+5)(d-6)\)

Try it.

\(\frac{3}{5{m}^{2}-3m-2},\frac{6m}{5{m}^{2}+17m+6}\)

In the following exercises, write as equivalent rational expressions with the given LCD.

Try it.

\(\frac{5}{{x}^{2}-2x-8},\frac{2x}{{x}^{2}-x-12}\)
LCD \((x-4)(x+2)(x+3)\)

Solution

\(\frac{5x+15}{(x-4)(x+2)(x+3)},\)
\(\frac{2{x}^{2}+4x}{(x-4)(x+2)(x+3)}\)

Try it.

\(\frac{8}{{y}^{2}+12y+35},\frac{3y}{{y}^{2}+y-42}\)
LCD \((y+7)(y+5)(y-6)\)

Try it.

\(\frac{9}{{z}^{2}+2z-8},\frac{4z}{{z}^{2}-4}\)
LCD \((z-2)(z+4)(z+2)\)

Solution

\(\frac{9z+18}{(z-2)(z+4)(z+2)},\)
\(\frac{4{z}^{2}+16z}{(z-2)(z+4)(z+2)}\)

Try it.

\(\frac{6}{{a}^{2}+14a+45},\frac{5a}{{a}^{2}-81}\)
LCD \((a+9)(a+5)(a-9)\)

Try it.

\(\frac{4}{{b}^{2}+6b+9},\frac{2b}{{b}^{2}-2b-15}\)
LCD \((b+3)(b+3)(b-5)\)

Solution

\(\frac{4b-20}{(b+3)(b+3)(b-5)},\)
\(\frac{2{b}^{2}+6b}{(b+3)(b+3)(b-5)}\)

Try it.

\(\frac{5}{{c}^{2}-4c+4},\frac{3c}{{c}^{2}-10c+16}\)
LCD \((c-2)(c-2)(c-8)\)

Try it.

\(\frac{2}{3{d}^{2}+14d-5},\frac{5d}{3{d}^{2}-19d+6}\)
LCD \((3d-1)(d+5)(d-6)\)

Solution

\(\frac{2d-12}{(3d-1)(d+5)(d-6)},\)
\(\frac{5{d}^{2}+25d}{(3d-1)(d+5)(d-6)}\)

Try it.

\(\frac{3}{5{m}^{2}-3m-2},\frac{6m}{5{m}^{2}+17m+6}\)
LCD \((5m+2)(m-1)(m+3)\)

In the following exercises, add.

Try it.

\(\frac{5}{24}+\frac{11}{36}\)

Solution

\(\frac{37}{72}\)

Try it.

\(\frac{7}{30}+\frac{13}{45}\)

Try it.

\(\frac{9}{20}+\frac{11}{30}\)

Solution

\(\frac{49}{60}\)

Try it.

\(\frac{8}{27}+\frac{7}{18}\)

Try it.

\(\frac{7}{10{x}^{2}y}+\frac{4}{15x{y}^{2}}\)

Solution

\(\frac{21y+8x}{30{x}^{2}{y}^{2}}\)

Try it.

\(\frac{1}{12{a}^{3}{b}^{2}}+\frac{5}{9{a}^{2}{b}^{3}}\)

Try it.

\(\frac{1}{2m}+\frac{7}{8{m}^{2}n}\)

Solution

\(\frac{4mn+7}{8{m}^{2}n}\)

Try it.

\(\frac{5}{6{p}^{2}q}+\frac{1}{4p}\)

Try it.

\(\frac{3}{r+4}+\frac{2}{r-5}\)

Solution

\(\frac{5r-7}{(r+4)(r-5)}\)

Try it.

\(\frac{4}{s-7}+\frac{5}{s+3}\)

Try it.

\(\frac{8}{t+5}+\frac{6}{t-5}\)

Solution

\(\frac{14t-10}{(t+5)(t-5)}\)

Try it.

\(\frac{7}{v+5}+\frac{9}{v-5}\)

Try it.

\(\frac{5}{3w-2}+\frac{2}{w+1}\)

Solution

\(\frac{11w+1}{(3w-2)(w+1)}\)

Try it.

\(\frac{4}{2x+5}+\frac{2}{x-1}\)

Try it.

\(\frac{2y}{y+3}+\frac{3}{y-1}\)

Solution

\(\frac{2{y}^{2}+y+9}{(y+3)(y-1)}\)

Try it.

\(\frac{3z}{z-2}+\frac{1}{z+5}\)

Try it.

\(\frac{5b}{{a}^{2}b-2{a}^{2}}+\frac{2b}{{b}^{2}-4}\)

Solution

\(\frac{b(5b+10+2{a}^{2})}{{a}^{2}(b-2)(b+2)}\)

Try it.

\(\frac{4}{cd+3c}+\frac{1}{{d}^{2}-9}\)

Try it.

\(\frac{2m}{3m-3}+\frac{5m}{{m}^{2}+3m-4}\)

Solution

\(\frac{2{m}^{2}+23m}{3(m-1)(m+4)}\)

Try it.

\(\frac{3}{4n+4}+\frac{6}{{n}^{2}-n-2}\)

Try it.

\(\frac{3}{{n}^{2}+3n-18}+\frac{4n}{{n}^{2}+8n+12}\)

Solution

\(\frac{4{n}^{2}-9n+6}{(n-3)(n+6)(n+2)}\)

Try it.

\(\frac{6}{{q}^{2}-3q-10}+\frac{5q}{{q}^{2}-8q+15}\)

Try it.

\(\frac{3r}{{r}^{2}+7r+6}+\frac{9}{{r}^{2}+4r+3}\)

Solution

\(\frac{3({r}^{2}+6r+18)}{(r+1)(r+6)(r+3)}\)

Try it.

\(\frac{2s}{{s}^{2}+2s-8}+\frac{4}{{s}^{2}+3s-10}\)

In the following exercises, subtract.

Try it.

\(\frac{t}{t-6}-\frac{t-2}{t+6}\)

Solution

\(\frac{2(7t-6)}{(t-6)(t+6)}\)

Try it.

\(\frac{v}{v-3}-\frac{v-6}{v+1}\)

Try it.

\(\frac{w+2}{w+4}-\frac{w}{w-2}\)

Solution

\(\frac{-4(1+w)}{(w+4)(w-2)}\)

Try it.

\(\frac{x-3}{x+6}-\frac{x}{x+3}\)

Try it.

\(\frac{y-4}{y+1}-\frac{1}{y+7}\)

Solution

\(\frac{{y}^{2}+2y-29}{(y+1)(y+7)}\)

Try it.

\(\frac{z+8}{z-3}-\frac{z}{z-2}\)

Try it.

\(\frac{5a}{a+3}-\frac{a+2}{a+6}\)

Solution

\(\frac{4{a}^{2}+25a-6}{(a+3)(a+6)}\)

Try it.

\(\frac{3b}{b-2}-\frac{b-6}{b-8}\)

Try it.

\(\frac{6c}{{c}^{2}-25}-\frac{3}{c+5}\)

Solution

\(\frac{3}{c-5}\)

Try it.

\(\frac{4d}{{d}^{2}-81}-\frac{2}{d+9}\)

Try it.

\(\frac{6}{m+6}-\frac{12m}{{m}^{2}-36}\)

Solution

\(\frac{-6}{m-6}\)

Try it.

\(\frac{4}{n+4}-\frac{8n}{{n}^{2}-16}\)

Try it.

\(\frac{-9p-17}{{p}^{2}-4p-21}-\frac{p+1}{7-p}\)

Solution

\(\frac{p+2}{p+3}\)

Try it.

\(\frac{-13q-8}{{q}^{2}+2q-24}-\frac{q+2}{4-q}\)

Try it.

\(\frac{-2r-16}{{r}^{2}+6r-16}-\frac{5}{2-r}\)

Solution

\(\frac{3}{r-2}\)

Try it.

\(\frac{2t-30}{{t}^{2}+6t-27}-\frac{2}{3-t}\)

Try it.

\(\frac{5v-2}{v+3}-4\)

Solution

\(\frac{v-14}{v+3}\)

Try it.

\(\frac{6w+5}{w-1}+2\)

Try it.

\(\frac{2x+7}{10x-1}+3\)

Solution

\(\frac{4(8x+1)}{10x-1}\)

Try it.

\(\frac{8y-4}{5y+2}-6\)

In the following exercises, add and subtract.

Try it.

\(\frac{5a}{a-2}+\frac{9}{a}-\frac{2a+18}{{a}^{2}-2a}\)

Solution

\(\frac{5{a}^{2}+7a-36}{a(a-2)}\)

Try it.

\(\frac{2b}{b-5}+\frac{3}{2b}-\frac{2b-15}{2{b}^{2}-10b}\)

Try it.

\(\frac{c}{c+2}+\frac{5}{c-2}-\frac{10c}{{c}^{2}-4}\)

Solution

\(\frac{c-5}{c+2}\)

Try it.

\(\frac{6d}{d-5}+\frac{1}{d+4}-\frac{7d-5}{{d}^{2}-d-20}\)

In the following exercises, simplify.

Try it.

\(\frac{6a}{3ab+{b}^{2}}+\frac{3a}{9{a}^{2}-{b}^{2}}\)

Solution

\(\frac{3a(6a-b)}{b(3a+b)(3a-b)}\)

Try it.

\(\frac{2c}{2c+10}+\frac{7c}{{c}^{2}+9c+20}\)

Try it.

\(\frac{6d}{{d}^{2}-64}-\frac{3}{d-8}\)

Solution

\(\frac{3}{d+8}\)

Try it.

\(\frac{5}{n+7}-\frac{10n}{{n}^{2}-49}\)

Try it.

\(\frac{4m}{{m}^{2}+6m-7}+\frac{2}{{m}^{2}+10m+21}\)

Solution

\(\frac{2(2{m}^{2}+7m-1)}{(m+7)(m-1)(m+3)}\)

Try it.

\(\frac{3p}{{p}^{2}+4p-12}+\frac{1}{{p}^{2}+p-30}\)

Try it.

\(\frac{-5n-5}{{n}^{2}+n-6}+\frac{n+1}{2-n}\)

Solution

\(-\frac{\left(n+1\right)\left(n+8\right)}{\left(n+3\right)\left(n-2\right)}\)

Try it.

\(\frac{-4b-24}{{b}^{2}+b-30}+\frac{b+7}{5-b}\)

Try it.

\(\frac{7}{15p}+\frac{5}{18pq}\)

Solution

\(\frac{42q+25}{90pq}\)

Try it.

\(\frac{3}{20{a}^{2}}+\frac{11}{12a{b}^{2}}\)

Try it.

\(\frac{4}{x-2}+\frac{3}{x+5}\)

Solution

\(\frac{7(x+2)}{(x-2)(x+5)}\)

Try it.

\(\frac{6}{m+4}+\frac{9}{m-8}\)

Try it.

\(\frac{2q+7}{q+4}-2\)

Solution

\(\frac{-1}{q+4}\)

Try it.

\(\frac{3y-1}{y+4}-2\)

Try it.

\(\frac{z+2}{z-5}-\frac{z}{z+1}\)

Solution

\(\frac{2\left(4z+1\right)}{(z-5)(z+1)}\)

Try it.

\(\frac{t}{t-5}-\frac{t-1}{t+5}\)

Try it.

\(\frac{3d}{d+2}+\frac{4}{d}-\frac{d+8}{{d}^{2}+2d}\)

Solution

\(\frac{3(d+1)}{d+2}\)

Try it.

\(\frac{2q}{q+5}+\frac{3}{q-3}-\frac{13q+15}{{q}^{2}+2q-15}\)

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

Now you Không máy tính nào có thể tính toán được nó, nhưng các mảnh của nó có thể tính toán được. Thử một trong những cách dưới đây, hoặc gõ theo ý bạn.

⌨ Kiểu ✎ Vẽ đi.

Câu hỏi toán của anh.

π Giải quyết

Luyện tập (40)

Đầu tiên thử mỗi câu trên giấy. Cho thấy câu trả lời để kiểm tra; những câu trả lời đã kiểm tra có thể mở trong trình giải cho mỗi bước.

1. Add: \(\frac{7}{10}+\frac{8}{15}.\)

   Làm việc đi. Gợi ý

   Giải đáp

   \(\frac{37}{30}\)

2. Subtract: \(6(2x+1)-4(x-5).\)

   Làm việc đi. Gợi ý

   Giải đáp

   \(8x+26\)

3. Find the Greatest Common Factor of \(9{x}^{2}{y}^{3}\) and \(12x{y}^{5}\).

   Làm việc đi. Gợi ý

   Giải đáp

   \(3x{y}^{3}\)

4. Factor completely \(-48n-12\).

   Làm việc đi. Gợi ý

   Giải đáp

   \(-12\left(4n+1\right)\)

5. Find the LCD for \(\frac{8}{{x}^{2}-2x-3},\frac{3x}{{x}^{2}+4x+3}\).

   Làm việc đi. Gợi ý

   Giải đáp

   	\(\text{Find the LCD for}\ \frac{8}{{x}^{2}-2x-3},\frac{3x}{{x}^{2}+4x+3}.\)
   Factor each expression completely, lining up common factors. Bring down the columns.	\(\frac{\begin{array}{l}{x}^{2}-2x-3=(x+1)(x-3) \\ {x}^{2}+4x+3=(x+1)\ (x+3)\end{array}}{\ \text{LCD}=(x+1)(x-3)(x+3)}\)
   Multiply the factors.	\(\text{The LCD is}\ (x+1)(x-3)(x+3).\)

6. Find the LCD for \(\frac{2}{{x}^{2}-x-12},\frac{1}{{x}^{2}-16}\).

   Làm việc đi. Gợi ý

   Giải đáp

   \((x-4)(x+4)(x+3)\)

7. Find the LCD for \(\frac{x}{{x}^{2}+8x+15},\frac{5}{{x}^{2}+9x+18}\).

   Làm việc đi. Gợi ý

   Giải đáp

   \((x+3)(x+6)(x+5)\)

8. Rewrite as equivalent rational expressions with denominator \((x+1)(x-3)(x+3)\): \(\frac{8}{{x}^{2}-2x-3},\frac{3x}{{x}^{2}+4x+3}.\)

   Làm việc đi. Gợi ý

   Giải đáp

   Factor each denominator.
   Find the LCD.
   Multiply each denominator by the 'missing' factor and multiply each numerator by the same factor.
   Simplify the numerators.

9. Rewrite as equivalent rational expressions with denominator \((x+3)(x-4)(x+4)\):
   \(\frac{2}{{x}^{2}-x-12},\frac{1}{{x}^{2}-16}.\)

   Làm việc đi. Gợi ý

   Giải đáp

   \(\frac{2x+8}{(x-4)(x+3)(x+4)},\)
   \(\frac{x+3}{(x-4)(x+3)(x+4)}\)

10. Rewrite as equivalent rational expressions with denominator \((x+3)(x+5)(x+6)\):
    \(\frac{x}{{x}^{2}+8x+15},\frac{5}{{x}^{2}+9x+18}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    \(\frac{{x}^{2}+6x}{(x+3)(x+5)(x+6)},\)
    \(\frac{5x+25}{(x+3)(x+5)(x+6)}\)

11. Add: \(\frac{7}{12}+\frac{5}{18}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    Find the LCD of 12 and 18.
    Rewrite each fraction as an equivalent fraction with the LCD.
    Add the fractions.
    The fraction cannot be simplified.

12. Add: \(\frac{11}{30}+\frac{7}{12}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    \(\frac{19}{20}\)

13. Add: \(\frac{3}{8}+\frac{9}{20}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    \(\frac{33}{40}\)

14. Add: \(\frac{5}{12{x}^{2}y}+\frac{4}{21x{y}^{2}}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    Find the LCD of \(12{x}^{2}y\) and \(21x{y}^{2}\).
    Rewrite each rational expression as an equivalent fraction with the LCD.
    Simplify.
    Add the rational expressions.
    There are no factors common to the numerator and denominator. The fraction cannot be simplified.

15. Add: \(\frac{2}{15{a}^{2}b}+\frac{5}{6a{b}^{2}}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    \(\frac{4b+25a}{30{a}^{2}{b}^{2}}\)

16. Add: \(\frac{5}{16c}+\frac{3}{8c{d}^{2}}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    \(\frac{5{d}^{2}+6}{16c{d}^{2}}\)

17. Add: \(\frac{3}{x-3}+\frac{2}{x-2}.\)

    Làm việc đi. Gợi ý

18. Add: \(\frac{2}{x-2}+\frac{5}{x+3}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    \(\frac{7x-4}{(x+3)(x-2)}\)

19. Add: \(\frac{4}{m+3}+\frac{3}{m+4}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    \(\frac{7m+25}{(m+3)(m+4)}\)

20. Add: \(\frac{2a}{2ab+{b}^{2}}+\frac{3a}{4{a}^{2}-{b}^{2}}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    Do the expressions have a common denominator? No. Rewrite each expression with the LCD.
    Find the LCD.
    Rewrite each rational expression as an equivalent rational expression with the LCD.
    Simplify the numerators.
    Add the rational expressions.
    Simplify the numerator.
    Factor the numerator.
    There are no factors common to the numerator and denominator. The fraction cannot be simplified.

21. Add: \(\frac{5x}{xy-{y}^{2}}+\frac{2x}{{x}^{2}-{y}^{2}}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    \(\frac{x(5x+7y)}{y(x-y)(x+y)}\)

22. Add: \(\frac{7}{2m+6}+\frac{4}{{m}^{2}+4m+3}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    \(\frac{7m+15}{2(m+3)(m+1)}\)

23. Add: \(\frac{8}{{x}^{2}-2x-3}+\frac{3x}{{x}^{2}+4x+3}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    Do the expressions have a common denominator? No. Rewrite each expression with the LCD.
    Find the LCD.
    Rewrite each rational expression as an equivalent fraction with the LCD.
    Simplify the numerators.
    Add the rational expressions.
    Simplify the numerator.
    The numerator is prime, so there are no common factors.

24. Add: \(\frac{1}{{m}^{2}-m-2}+\frac{5m}{{m}^{2}+3m+2}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    \(\frac{5{m}^{2}-9m+2}{(m-2)(m+1)(m+2)}\)

25. Add: \(\frac{2n}{{n}^{2}-3n-10}+\frac{6}{{n}^{2}+5n+6}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    \(\frac{2({n}^{2}+6n-15)}{(n+2)(n-5)(n+3)}\)

26. Subtract: \(\frac{x}{x-3}-\frac{x-2}{x+3}.\)

    Làm việc đi. Gợi ý

27. Subtract: \(\frac{y}{y+4}-\frac{y-2}{y-5}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    \(\frac{-7y+8}{(y+4)(y-5)}\)

28. Subtract: \(\frac{z+3}{z+2}-\frac{z}{z+3}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    \(\frac{4z+9}{(z+2)(z+3)}\)

29. Subtract: \(\frac{8y}{{y}^{2}-16}-\frac{4}{y-4}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    Do the expressions have a common denominator? No. Rewrite each expression with the LCD.
    Find the LCD.
    Rewrite each rational expression as an equivalent rational expression with the LCD.
    Simplify the numerators.
    Subtract the rational expressions.
    Simplify the numerators.
    Factor the numerator to look for common factors.
    Remove common factors.
    Simplify.

30. Subtract: \(\frac{2x}{{x}^{2}-4}-\frac{1}{x+2}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    \(\frac{1}{x-2}\)

31. Subtract: \(\frac{3}{z+3}-\frac{6z}{{z}^{2}-9}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    \(\frac{-3}{z-3}\)

32. Subtract: \(\frac{-3n-9}{{n}^{2}+n-6}-\frac{n+3}{2-n}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    Factor the denominator.
    Since \(n-2\) and \(2-n\) are opposites, we will mutliply the second rational expression by\(\frac{-1}{-1}\).
    Simplify.
    Do the expressions have a common denominator? No.
    Find the LCD.
    Rewrite each rational expression as an equivalent rational expression with the LCD.
    Simplify the numerators.
    Simplify the rational expressions.
    Simplify the numerator.
    Factor the numerator to look for common factors.
    Simplify.

33. Subtract: \(\frac{3x-1}{{x}^{2}-5x-6}-\frac{2}{6-x}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    \(\frac{5x+1}{\left(x-6\right)\left(x+1\right)}\)

34. Subtract: \(\frac{-2y-2}{{y}^{2}+2y-8}-\frac{y-1}{2-y}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    \(\frac{y+3}{y+4}\)

35. Subtract: \(\frac{5c+4}{c-2}-3.\)

    Làm việc đi. Gợi ý

    Giải đáp

    Write \(3\) as \(\frac{3}{1}\) to have 2 rational expressions.
    Do the rational expressions have a common denominator? No.
    Find the LCD of \(c-2\) and \(1.\) LCD = \(c-2.\)
    Rewrite \(\frac{3}{1}\) as an equivalent rational expression with the LCD.
    Simplify.
    Subtract the rational expressions.
    Simplify.
    Factor to check for common factors.
    There are no common factors; the rational expression is simplified.

36. Subtract: \(\frac{2x+1}{x-7}-3.\)

    Làm việc đi. Gợi ý

    Giải đáp

    \(\frac{\text{-}x+22}{x-7}\)

37. Subtract: \(\frac{4y+3}{2y-1}-5.\)

    Làm việc đi. Gợi ý

    Giải đáp

    \(\frac{-2(3y-4)}{2y-1}\)

38. Simplify: \(\frac{2u}{u-1}+\frac{1}{u}-\frac{2u-1}{{u}^{2}-u}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    Do the rational expressions have a common denominator? No.
    Find the LCD.
    Rewrite each rational expression as an equivalent rational expression with the LCD.
    Write as one rational expression.
    Simplify.
    Factor the numerator, and remove common factors.
    Simplify.

39. Simplify: \(\frac{v}{v+1}+\frac{3}{v-1}-\frac{6}{{v}^{2}-1}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    \(\frac{v+3}{v+1}\)

40. Simplify: \(\frac{3w}{w+2}+\frac{2}{w+7}-\frac{17w+4}{{w}^{2}+9w+14}.\)

    Làm việc đi. Gợi ý

    Giải đáp

    \(\frac{3w}{w+7}\)

Để cho anh làm việc

Một tài khoản miễn phí thêm ghi chú vào mỗi bài học, ghi chép những gì bạn đã hoàn thành, các bài toán bạn giải quyết ở một nơi, và một giáo viên bạn có thể hỏi về trang này. Toán học là mở cho mọi người, dù có đăng nhập hay không.

Đăng ký Đăng nhập

Ký hiệu được dùng ở đây

Nhấn vào bất kỳ ký hiệu nào để xem định nghĩa đầy đủ, một bức ảnh, và ý nghĩa của từng chữ cái trong nó.

f'(x)derivative
Instantaneous rate of change; slope of the graph.\leqless/greater than or equal
Inequalities that allow equality; < and > exclude it.\pmplus or minus
Both signs at once: x = 3 ± 2 means 5 and 1.\neqnot equal
The two sides are different.\sqrt{x}square root, n-th root
The non-negative number whose square (n-th power) is x.|x|absolute value / modulus
Distance from zero: |−3| = 3. For a complex number, distance from the origin.iimaginary unit
i² = −1.\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.

Làm thế nào: Add and Subtract Rational Expressions with Unlike Denominators

1. Find the least common denominator of rational expressions
2. Find equivalent rational expressions
3. Add rational expressions with different denominators
4. Subtract rational expressions with different denominators
5. Factor each expression completely.
6. List the factors of each expression. Match factors vertically when possible.
7. Bring down the columns.
8. Multiply the factors.

Câu hỏi người ta hỏi

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.

Một số phần của trang này được chuyển từ OpenStax Elementary Algebra 2e (CC BY-NC-SA 4.0). Được nén lại và giải thích lại ở đây; lỗi là của chúng tôi. Bởi vì nguồn của nó được cấp phép cho sử dụng không thương mại, bài học này là miễn phí, không bán gì, và được chia sẻ dưới cùng một giấy phép.

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