sinx - sin3x = 4sin²x·cosx
2sin((x-3x)/2)·cos((x + 3x)/2) = 4sin²x·cosx
2sin(-x)·cos2x = 4sin²x·cosx
-2sinx·cos2x = 4sin²x·cosx
-cos2x = 2sinx·cosx
-cos2x = sin2x
tg2x = -1
2x = arctg (-1) + πn
2x = -π/4 + πn
x = -π/8 + πn/2 n∈Z
