A particle moves along the $x$-axis so that its velocity is given by $v(t)=5+3.1\cos(0.8t)$ for $t\geq 0$. What is the acceleration of the particle at $t=4$.
I differentiated this and got:
$a(t)=-2.48 \sin(0.8t) $
and plugged in $4$ and got
$a(4) = -0.138437$
But none of the answer choices fit the answer, am I wrong or is the question wrong?
Answer choices: $-0.045$, $-0.145$, $-0.181$, $0.145$, $5.145$