\int_{0}^{\pi/2}{\frac{dx}{(a^2cos^2x+b^2sin^2x)}}=\frac{\pi}{2ab}\\
differntiate the given eq firstly wrt to a....i
and then wrt to b....ii
add them i.e i and ii and then see wat happens.
u will get the resulting integral as \frac{\pi(a^2+b^2)}{4a^3b^3}

