Precalculus (6th Edition) Blitzer

Published by Pearson
ISBN 10: 0-13446-914-3
ISBN 13: 978-0-13446-914-0

Chapter P - Section P.6 - Rational Expressions - Exercise Set - Page 86: 89

Answer

The required solution is: $\frac{2d}{\left( {{a}^{2}}+ab+{{b}^{2}} \right)}$.

Work Step by Step

We have the given algebraic expression $\left( \frac{1}{{{a}^{3}}-{{b}^{3}}}\cdot \frac{ac+ad-bc-bd}{1} \right)-\frac{c-d}{{{a}^{2}}+ab+{{b}^{2}}}$. Solve the bracket of the given expression: $\frac{1}{{{a}^{3}}-{{b}^{3}}}\cdot \frac{ac+ad-bc-bd}{1}$. Now factorize ${{a}^{3}}-{{b}^{3}}$ by using the identity ${{x}^{3}}-{{y}^{3}}=\left( x-y \right)\left( {{x}^{2}}+xy+{{y}^{2}} \right)$: $\begin{align} & {{a}^{3}}-{{b}^{3}}={{\left( a \right)}^{3}}-{{\left( b \right)}^{3}} \\ & =\left( a-b \right)\left( {{a}^{2}}+ab+{{b}^{2}} \right) \end{align}$. Factorize $ac+ad-bc-bd$ by taking out common terms: $\begin{align} & ac+ad-bc-bd=a\left( c+d \right)-b\left( c+d \right) \\ & =\left( c+d \right)\left( a-b \right) \end{align}$. Therefore, the bracket of the given expression gets reduced to $\begin{align} & \frac{1}{{{a}^{3}}-{{b}^{3}}}\cdot \frac{ac+ad-bc-bd}{1}=\frac{1}{\left( a-b \right)\left( {{a}^{2}}+ab+{{b}^{2}} \right)}\cdot \frac{\left( c+d \right)\left( a-b \right)}{1} \\ & =\frac{\left( c+d \right)}{\left( {{a}^{2}}+ab+{{b}^{2}} \right)} \end{align}$. Simplify the given algebraic expression: $\begin{align} & \left( \frac{1}{{{a}^{3}}-{{b}^{3}}}\cdot \frac{ac+ad-bc-bd}{1} \right)-\frac{c-d}{{{a}^{2}}+ab+{{b}^{2}}}=\frac{\left( c+d \right)}{\left( {{a}^{2}}+ab+{{b}^{2}} \right)}-\frac{c-d}{{{a}^{2}}+ab+{{b}^{2}}} \\ & =\frac{c+d-c+d}{\left( {{a}^{2}}+ab+{{b}^{2}} \right)} \\ & =\frac{2d}{\left( {{a}^{2}}+ab+{{b}^{2}} \right)} \end{align}$. Therefore, $\left( \frac{1}{{{a}^{3}}-{{b}^{3}}}\cdot \frac{ac+ad-bc-bd}{1} \right)-\frac{c-d}{{{a}^{2}}+ab+{{b}^{2}}}=$ $\frac{2d}{\left( {{a}^{2}}+ab+{{b}^{2}} \right)}$.
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