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Bài 1: CMR
$(\frac{\tan a+\cos a}{1+\cot a.\cos a})^{n}=\frac{\tan ^{n}a+\cos ^{n}a}{1+\cot ^{n}a.\cos ^{n}a}$
Bài 2: Tính tổng
a/ $S_{n}=\frac{1}{\sin a}+\frac{1}{\sin 2a}+...+\frac{1}{\sin2^{n-1}a}$
b/ $S_{n}=\sin^{3}\frac{a}{3}+3\sin^{3}\frac{a}{3^{2}}+...+3^{n-1}.\sin^{3}\frac{a}{3^{n}}$
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