[JamesCoe#18]

$\int\frac{1}{\sqrt{t}}cos^2(\sqrt{t}+3)$

$ให้ u=\sqrt{t}+3$

$dt=2\sqrt{t}du$

$\int2cos^2udu$

$\int\cos2u+1$

$\frac{sin2(\sqrt{t}+3)}{2}+(\sqrt{t}+3)+C$