<meta http-equiv="refresh" content="1; url=/nojavascript/"> Solving Rational Equations using Cross-Multiplication ( Read ) | Algebra | CK-12 Foundation
Dismiss
Skip Navigation
You are viewing an older version of this Concept. Go to the latest version.

Solving Rational Equations using Cross-Multiplication

%
Best Score
Practice
Best Score
%
Practice Now

Solving Rational Equations using Cross-Multiplication

A scale model of a racecar is in the ratio of 1: x to the real racecar. The length of the model is 2x-21 units, and the length of the real racecar is x^2 units. What is the value of x ?

Guidance

A rational equation is an equation where there are rational expressions on both sides of the equal sign. One way to solve rational equations is to use cross-multiplication. Here is an example of a proportion that we can solve using cross-multiplication.

If you need more of a review of cross-multiplication, see the Proportion Properties concept in the Geometry FlexBook. Otherwise, we will start solving rational equations using cross-multiplication.

Example A

Solve \frac{x}{2x-3}=\frac{3x}{x+11} .

Solution: Use cross-multiplication to solve the problem. You can use the example above as a guideline.

Check your answers. It is possible to get extraneous solutions with rational expressions.

\frac{0}{2 \cdot 0-3}&=\frac{3 \cdot 0}{0+11} && \frac{4}{2 \cdot 4-3}=\frac{3 \cdot 4}{4+11} \\\frac{0}{-3}&=\frac{0}{11} \  && \qquad \quad \frac{4}{5}=\frac{12}{15} \\0&=0 && \qquad \quad \frac{4}{5}=\frac{4}{5}

Example B

Solve \frac{x+1}{4}=\frac{3}{x-3} .

Solution: Cross-multiply and solve.

\frac{x+1}{4}&=\frac{3}{x-3} \\12&=x^2-2x-3 \\0&=x^2-2x-15 \\0&=(x-5)(x+3) \\x&=5 \ and \ -3

Check your answers.

\frac{5+1}{4}=\frac{3}{5-3} \rightarrow \frac{6}{4}=\frac{3}{2} and

\frac{-3+1}{4}=\frac{3}{-3-3} \rightarrow \frac{-2}{4}=\frac{3}{-6}

Example C

Solve \frac{x^2}{2x-5}=\frac{x+8}{2} .

Solution: Cross-multiply.

\frac{x^2}{2x-5}&=\frac{x+8}{2} \\2x^2+11x-40&=2x^2 \\11x-40&=0 \\11x&=40 \\x&=\frac{40}{11}

Check the answer: \frac{\left(\frac{40}{11}\right)^2}{\frac{80}{11}-5}=\frac{\frac{40}{11}+8}{2} \rightarrow \frac{1600}{121} \div \frac{25}{11}=\frac{128}{11} \div 2 \rightarrow \frac{64}{11}=\frac{128}{22}

Intro Problem Revisit We need to set up a rational equation and solve for x .

\frac{1}{x} = \frac{2x-21}{x^2}

Now cross-multiply.

x^2 = x(2x-21)\\x^2 = 2x^2 - 21x\\0 = x^2 - 21x\\0 = x(x - 21)\\x = 0, 21

However, x is a ratio so it must be greater than 0. Therefore x equals 21 and the model is in the ratio 1:21 to the real racecar.

Guided Practice

Solve the following rational equations.

1. \frac{-x}{x-1}=\frac{x-8}{3}

2. \frac{x^2-1}{x+2}=\frac{2x-1}{2}

3. \frac{9-x}{x^2}=\frac{4}{3x}

Answers

1. \frac{-x}{x-1}&=\frac{x-8}{3} \\x^2-9x+8&=-3x \\x^2-6x+8&=0 \\(x-4)(x-2)&=0\\x&=4 \ and \ 2

\text{Check}: x=4 \rightarrow \frac{-4}{4-1}&=\frac{4-8}{3} && x=2 \rightarrow \frac{-2}{2-1}=\frac{2-8}{3} \\\frac{-4}{3}&=\frac{-4}{3} && \qquad \qquad \quad \frac{-2}{1}=\frac{-6}{3}

2. \frac{x^2-1}{x+2}&=\frac{2x-1}{2} \\2x^2+3x-2&=2x^2-2\\3x&=0 \\x&=0

\text{Check}: \frac{0^2-1}{0+2}&=\frac{2 \left(0\right)-1}{2} \\\frac{-1}{2}&=\frac{-1}{2}

3. \frac{9-x}{x^2}&=\frac{4}{-3x} \\4x^2&=-27x+3x^2 \\x^2+27x&=0 \\x(x+27)&=0 \\x&=0 \ and \ -27

\text{Check}: x=0 \rightarrow \frac{9-0}{0^2}&=\frac{4}{-3 \left(0\right)} && x=-27 \rightarrow \frac{9+27}{\left(-27\right)^2}=\frac{4}{-3 \left(-27\right)} \\und&=und && \qquad \qquad \qquad \quad \frac{36}{729}=\frac{4}{81} \\& && \qquad \qquad \qquad \quad \ \frac{4}{81}=\frac{4}{81}

x = 0 is not actually a solution because it is a vertical asymptote for each rational expression, if graphed. Because zero is not part of the domain, it cannot be a solution, and is extraneous.

Vocabulary

Rational Equation
An equation where there are rational expressions on both sides of the equal sign.

Problem Set

  1. Is x=-2 a solution to \frac{x-1}{x-4}=\frac{x^2-1}{x+4} ?

Solve the following rational equations.

  1. \frac{2x}{x+3}=\frac{8}{x}
  2. \frac{4}{x+1}=\frac{x+2}{3}
  3. \frac{x^2}{x+2}=\frac{x+3}{2}
  4. \frac{3x}{2x-1}=\frac{2x+1}{x}
  5. \frac{x+2}{x-3}=\frac{x}{3x-2}
  6. \frac{x+3}{-3}=\frac{2x+6}{x-3}
  7. \frac{2x+5}{x-1}=\frac{2}{x-4}
  8. \frac{6x-1}{4x^2}=\frac{3}{2x+5}
  9. \frac{5x^2+1}{10}=\frac{x^3-8}{2x}
  10. \frac{x^2-4}{x+4}=\frac{2x-1}{3}

Determine the values of a that make each statement true. If there no values, write none.

  1. \frac{1}{x-a}=\frac{x}{x+a} , such that there is no solution.
  2. \frac{1}{x-a}=\frac{x}{x-a} , such that there is no solution.
  3. \frac{x-a}{x}=\frac{1}{x+a} , such that there is one solution.
  4. \frac{1}{x+a}=\frac{x}{x-a} , such that there are two integer solutions.

Image Attributions

Reviews

Email Verified
Well done! You've successfully verified the email address .
OK
Please wait...
Please wait...

Original text