B
be_mum_mim


Đơn giản biểu thức
$\frac{1}{cos\alpha.cos2\alpha}+\frac{1}{cos2\alpha.cos3\alpha}+...+\frac{1}{cos7\alpha.cos8\alpha}$
$\frac{1}{cos\alpha.cos2\alpha}+\frac{1}{cos2\alpha.cos3\alpha}+...+\frac{1}{cos7\alpha.cos8\alpha}$