Calculus with Applications (10th Edition)

Published by Pearson
ISBN 10: 0321749006
ISBN 13: 978-0-32174-900-0

Chapter 3 - The Derivative - 3.2 Continuity - 3.2 Exercises - Page 147: 28

Answer

$$ h(x)=\left\{\begin{array}{ll}{\frac{3 x^{2}+2 x-8}{x+2}} & {\text { if } x \neq-2} \\ {3 x+k} & {\text { if } x=-2}\end{array}\right. $$ The value of the constant $k$ that makes the function continuous is . $k=-4 $

Work Step by Step

$$ h(x)=\left\{\begin{array}{ll}{\frac{3 x^{2}+2 x-8}{x+2}} & {\text { if } x \neq-2} \\ {3 x+k} & {\text { if } x=-2}\end{array}\right. $$ This rational function is discontinuous wherever the denominator is zero. There is a discontinuity when $2=-2$ . We investigate at $x=-2$ first. $$ \begin{aligned} \lim _{x \rightarrow -2} h(x) &=\lim _{x \rightarrow -2}\frac{3 x^{2}+2 x-8}{x+2} \\ &=\lim _{x \rightarrow -2}\frac{(3 x-4)(x+2)}{(x+2)} \\ &=\lim _{x \rightarrow -2}(3 x-4)\\ &=3 (-2)-4 \\ &=-10 \end{aligned} $$ Furthermore, $$ h(-2)= 3(-2)+k=-6+k $$ since, $h$ is continuous, then we have $$ h(-2)=\lim _{x \rightarrow -2} h(x) $$ so we obtain $$ \begin{aligned} -6+k &=-10\\ k &=-10+6 \\ -4 &=k \end{aligned} $$ Thus the value of the constant $k$ that makes the function continuous is . $k=-4$
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