# Show that two integrals are equal:

Can someone show how to get the equality $$\int_0^{\pi/2} \cos^m(x) \sin^m(x) dx = 2^{-m}\int_0^{\pi/2} \cos^m (x) dx$$ with integral substitution? Thanks!

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Hints: 1) $\sin 2x=2\sin x \cos x$ and 2) $\sin(\pi/2-y)=\cos y$. – Start wearing purple Aug 6 '13 at 10:30
@O.L.: Make it him as an answer. :) – Babak S. Aug 6 '13 at 10:57
Oh man, I feel dumb now...I totally forgot about trig relations and was just trying u-sub with sines and cosines. Thanks for the tip! – paul Aug 6 '13 at 10:57

$$\int_0^{\frac\pi2}\sin^mx\cos^mxdx=\frac1{2^m} \int_0^{\frac\pi2}(\sin2x)^mdx$$
$$=\frac1{2^m} \int_0^{\pi}(\sin y)^m\frac{dy}2$$
$$=\frac1{2^m} \int_0^{\frac\pi2}(\sin y)^m\frac{dy}2+\frac1{2^m} \int_{\frac\pi2}^\pi(\sin y)^m\frac{dy}2$$
Put $y=x-\frac\pi2$ and use $$\int_a^bf(x)dx=\int_a^bf(a+b-x)dx\text{ to find }\int_0^{\frac\pi2}(\sin y)^mdy=\int_0^{\frac\pi2}(\cos y)^mdy$$