High School Math : Understanding Rational Expressions

Study concepts, example questions & explanations for High School Math

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Example Questions

Example Question #1 : Rational Expressions

Solve:

If \displaystyle y varies directly as \displaystyle x, and \displaystyle x=7 when \displaystyle y=21, find \displaystyle x when \displaystyle y=-5.

Possible Answers:

\displaystyle x=\frac{-7}{3}

\displaystyle x=\frac{-11}{3}

\displaystyle x=\frac{-13}{3}

\displaystyle x=\frac{-1}{3}

\displaystyle x=\frac{-5}{3}

Correct answer:

\displaystyle x=\frac{-5}{3}

Explanation:

The formula for a direct variation is:

\displaystyle \frac{x_1}{x_2}=\frac{y_1}{y_2}

Plugging in our values, we get:

\displaystyle \frac{7}{x}=\frac{21}{-5}

\displaystyle 21x=-35

\displaystyle x= \frac{-5}{3}

Example Question #2 : Rational Expressions

If two boxes have the same depth and capacity, the length is inversely proportional to the width. One box is \displaystyle 60\ cm long and \displaystyle 40\ cm wide. A second box (same depth and capacity) is \displaystyle 5\ cm long. How wide is it?

 

Possible Answers:

\displaystyle 460\ cm

\displaystyle 450\ cm

\displaystyle 470\ cm

 

\displaystyle 490\ cm

\displaystyle 480\ cm

Correct answer:

\displaystyle 480\ cm

Explanation:

The formula for an indirect variation is:

\displaystyle \frac{x_1}{x_2}=\frac{y_2}{y_1}

Plugging in our values, we get:

\displaystyle \frac{60\ cm}{5\ cm}=\frac{y}{40\ cm}

\displaystyle 5y=2400\ cm

\displaystyle y=480\ cm

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