Biến đổi thành tích: A= cosx + cos3x + cos5x + cos7x.
Giải thích
A = cosx + cos3x + cos5x + cos7x
= 2cos4x.cos3x + 2cos4x.cosx
= 2cos4x.(cos3x + cosx)
= 4cos4x.cos2x.cosx
A = cosx + cos3x + cos5x + cos7x
= 2cos4x.cos3x + 2cos4x.cosx
= 2cos4x.(cos3x + cosx)
= 4cos4x.cos2x.cosx