cos3x - cos5x = cosx
suy ra cos(3x) = cosx + cos(5x)
suy ra cos(3x) = 2.cos(3x).cos(2x)
suy ra cos(3x).(1 - 2cos(2x)) = 0
suy ra cos(3x) = 0 hoặc cos(2x) = 1/2
với cos(3x) = 0 suy ra x = pi/6 + kpi/3
với cos(2x) = 1/2 suy ra x = ± pi/6 + kpi/2