Câu 1:
A = 2sinxsin3x
    = 2. (1/2).(cos2x - cos4x)
    = cos2x - cos4x
=> chọn C
Câu 2:
B = 8cosxsin2xsin3x
    = 8cosx.(1/2).(cosx - cos5x)
    = 4cos^2(x) - 4cosxcos5x
    = 2(1 + cos2x) - 2(cos4x + cos6x)
    = - 2cos6x - 2cos4x + 2cos2x + 2
=> chọn A
Câu 3:
C = sin(x + 15).cos(x - 15)
    = (1/2).(sin30 + sin2x)
    = 1/4 + 1/2.sin2x