Tính các tích phân sau :
a) $I = \int\limits_{0}^{\frac{\pi}{4} } \frac{\sin 4xdx}{\sin ^6x + \cos ^6x}$
b) $J = \int\limits_{0}^{\frac{\pi}{4} } \log_2(1+\tan x)dx$
c) $K = \int\limits_{\frac{\pi}{6} }^{\frac{\pi}{3} } \frac{dx}{\sin x.\sin \left ( x+\frac{\pi}{4} \right ) }.$
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