Question

Find the average value of the function f(t)=cos13(5t)sin(5t) f(t)=cos13⁡(5t)sin⁡(5t) on the interval [4,10].

Answer 1

The average value is given by

$\displaystyle\frac1{10-4}\int_4^{10}\cos^{13}5t\sin5t\,\mathrm dt$

Setting $y=\cos5t$, you get $\mathrm dy=-5\sin5t\,\mathrm dt$, and the integral becomes

$\displaystyle-\frac1{30}\int_{\cos20}^{\cos50}y^{13}\,\mathrm dy$
$=-\dfrac1{30}\dfrac{y^{14}}{14}\bigg|_{y=\cos20}^{y=\cos50}$
$=-\dfrac{\cos^{14}50-\cos^{14}20}{420}\approx-0.00145$