2cos^3x=sin(5пи/2 - x)
2cos^3x=sin(5Π/2-x) 2cos^3x=cosx 2cos^3x-cosx=0 | : на cosx 2cos^2x-1=0 2cos^2x=1 cos^2x=1/2 cosx=+-√2/2 x=-Π/4+2Πn, n€Z x=Π/4+2Πn, n€Z