College Algebra 7th Edition

Published by Brooks Cole
ISBN 10: 1305115546
ISBN 13: 978-1-30511-554-5

Chapter 5, Systems of Equations and Inequalities - Section 5.3 - Partial Fractions - 5.3 Exercises - Page 462: 26

Answer

$\frac{-3x^2-3x+27}{(x+2)(x+3)(2x-3)}=\frac{-3}{x+2}+\frac{1}{x+3}+\frac{1}{2x-3}$,

Work Step by Step

$\frac{-3x^2-3x+27}{(x+2)(2x^2+3x-9)}$, factorising $2x^2+3x-9=(x+3)(2x-3)$: $\frac{-3x^2-3x+27}{(x+2)(x+3)(2x-3)}=\frac{A}{x+2}+\frac{B}{x+3}+\frac{C}{2x-3}$, $-3x^2-3x+27=A(x+3)(2x-3)+B(x+2)(2x-3)+C(x+2)(x+3)$, $-3x^2-3x+27=A(2x^2+3x-9)+B(x^2+5x+6)+C(2x^2+x-6)$, $-3x^2-3x+27=(2A+B+2C)x^2+(3A+5B+C)x+(-9A+6B-6C)=-3x^2-3x+27$ $\begin{cases} & 2A & +B & +2C= -3&\\ & 3A & +5B & +C= -3&\\ & -9A & +6B & -6C =27&\\ \end{cases}$ Multiplying Equation $1$ by $-\frac{3}{2}$ and adding it to Equation $2$, $\begin{cases} & -3A & -\frac{3}{2}B & -3C =\frac{9}{2}\\ & 3A & +5B & +C = -3\\ &-- -- & -- -- & -- -- &\\ & +0A & \frac{7}{2}B & -2C = \frac{3}{2} \end{cases}$ $7B -4C=3$, multiplying both sided by $2$. Multiplying Equation $2$ by $3$ and Adding it to Equation $3$, $\begin{cases} & 9A & +15B & +3C =- 9\\ & -9A & +6B & -6C = 27\\ & -- -- & -- -- & -- -- &\\ & 0A & 21B & -3C = 18 \end{cases}$ $7B-C=6$, dividing both sides by $3$, Multiplying the result of the first Addition by $-1$ and Adding it to the result of the second addition: $\begin{cases} & -7B & +4C = -3\\ & 7B & -C =6\\ & -- -- & -- --& \\ & 0B & 3C = 3 \end{cases}$ thus, $C=1$ Substituting back $C=1$ into the equation, $7B-1=6, B=1$. Substituting $B$ and $C$ into the first Equation, $2A+1+2=-3, A=-3$. Therefore, $\frac{-3x^2-3x+27}{(x+2)(x+3)(2x-3)}=\frac{-3}{x+2}+\frac{1}{x+3}+\frac{1}{2x-3}$,
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