Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

Published by Pearson
ISBN 10: 0-32193-104-1
ISBN 13: 978-0-32193-104-7

Chapter 10 - Systems of Equations and Inequalities - Section 10.5 Partial Fraction Decomposition - 10.5 Assess Your Understanding - Page 788: 31

Answer

$\dfrac{x}{(3x-2)(2x+1)}=\dfrac{\frac{2}{7}}{3x-2}+\dfrac{\frac{1}{7}}{2x+1}$

Work Step by Step

We can notice from the given rational expression $\dfrac{A(x)}{B(x)}$ that $B(x)$ has non-repeated irreducible quadratic factor. So, we will write the given rational expression into partial fraction form as: $$\displaystyle \frac{x}{(3x-2)(2x+1)}=\frac{A}{3x-2}+\frac{B}{2x+1} …(1)$$ Now, write the RHS with a common denominator. $$\displaystyle \frac{x}{(3x-2)(2x+1)}=\dfrac{A(2x+1)+B(3x-2)}{(3x-2)(2x+1)} \\ \displaystyle \frac{x}{(3x-2)(2x+1)}=\dfrac{2Ax+A+3Bx-2B}{(3x-2)(2x+1)}\\ \displaystyle \frac{x}{(3x-2)(2x+1)}=\dfrac{(2A+3B)x+(A-2B)}{(3x-2)(2x+1)} $$ Next, equate the numerators to obtain: $$(2A+3B)x+(A-2B)=x$$ Equate the like coefficients of the polynomials on the LHS and RHS to obtain: $$2A+3B=1 ~~~(2) \\ A-2B=0 ~~~(3)$$ Equation (3) yields: $$-2B=-A \\ B=\dfrac{A}{2} ~~~(4) $$ Substitute $B=\dfrac{A}{2}$ into equation (2) to solve for $A$. $$2A+3(\dfrac{A}{2}) =1 \\ 2A+\dfrac{3A}{2}=1 \\ \dfrac{4A+3A}{2}=1\\ \dfrac{7}{2} A=1\\ A=\dfrac{2}{7}$$ Finally, substitute $A=\dfrac{2}{7}$ into equation (4) to solve for $B$. $$B=\dfrac{A}{2} \\ B=\dfrac{2}{14}=\dfrac{1}{7} $$ Therefore, the equation (1) becomes: $\displaystyle \dfrac{x}{(3x-2)(2x+1)}=\dfrac{\frac{2}{7}}{3x-2}+\dfrac{\frac{1}{7}}{2x+1}$
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