Answer
$$\sqrt{21}$$
Work Step by Step
We have $r'(t) =\lt 2,4,-1\gt$. Hence the speed $v(t)$ is given by
$$v(t)=\|r'(t)\|=\sqrt{4+16+1}=\sqrt{21}. $$ At $t=4$, $v(4)=\sqrt{21}$
Calculus (3rd Edition)
by Rogawski, Jon; Adams, Colin
Published by
W. H. Freeman
ISBN 10:
1464125260
ISBN 13:
978-1-46412-526-3
Chapter 14 - Calculus of Vector-Valued Functions - 14.3 Arc Length and Speed - Exercises - Page 725: 11
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