Bài 1: Giả sử:
$\frac{cosa+cosb+cosc}{cos(a+b+c)} =\frac{sina + sinb + sinc}{sin(a+b+c)}=m$
CMR : $cos(a+b)+cos(b+c)+cos(c+a) = m$

bị xóa vì bị trùng bởi Đức Vỹ 27-10-13 12:58 PM

Câu hỏi này có nội dung hoàn toàn trùng với câu hỏi đã có.

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