$\begin{aligned}
& \int \frac{\sec ^2 x d x}{a^2 \tan ^2 x+b^2} \
& \text { let } \tan x=t \
& \sec ^2 d x=d t \
& \int \frac{d t}{a^2 t^2+b^2} \
& \frac{1}{a^2} \int \frac{d t}{t^2+\left(\frac{b}{a}\right)^2} \
& \frac{1}{a^2} \frac{1}{\frac{b}{a}} \tan ^{-1}\left(\frac{t}{b} a\right)+c \
& \frac{1}{a b} \tan ^{-1}\left(\frac{\alpha}{b} \tan x\right)+c
\end{aligned}oncomparing\frac{\mathrm{a}}{\mathrm{b}}=3\begin{aligned}
& a b=12 \
& a=6, b=2
\end{aligned}maximumvalueof6 \sin x+2 \cos x \text { is } \sqrt{40}$