Calculus (3rd Edition)

Published by W. H. Freeman
ISBN 10: 1464125260
ISBN 13: 978-1-46412-526-3

Chapter 2 - Limits - 2.5 Evaluating Limits Algebraically - Exercises - Page 72: 34

Answer

$$ \lim _{x \rightarrow \frac{\pi }{3}} \frac{2 \cos ^{2} x+3 \cos x-2}{2 \cos x-1} = \frac{5 }{2} $$

Work Step by Step

Given $$ \lim _{x \rightarrow \frac{\pi }{3}} \frac{2 \cos ^{2} x+3 \cos x-2}{2 \cos x-1}$$ let $$ f(x) = \frac{2 \cos ^{2} x+3 \cos x-2}{2 \cos x-1}$$ Since, we have $$ f(\frac{\pi }{3})= \frac{2 \cos ^{2} \frac{\pi }{3}+3 \cos \frac{\pi }{3}-2}{2 \cos \frac{\pi }{3}-1}= \frac{ \frac{1}{2}+ \frac{3 }{2}-2}{1-1}=\frac{0}{0}$$ So, transform algebraically and cancel, we get \begin{aligned} L&=\lim _{x \rightarrow \frac{\pi }{3}} \frac{2 \cos ^{2} x+3 \cos x-2}{2 \cos x-1}\\ &= \lim _{x \rightarrow \frac{\pi }{3}} \frac{(2 \cos x-1) ( \cos x+2)}{2 \cos x-1}\\ &= \lim _{x \rightarrow \frac{\pi }{3}} ( \cos x+2)\\ &= ( \cos \frac{\pi }{3}+2) \\ &= \frac{1 }{2}+2 \\ &= \frac{5 }{2} \\ \end{aligned}
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