sin^3 x (1+ctg x) + cos^3 x (1+tg x)= sin^3x + sin^2x * cosx + cos^3x + sinx * cos^2x = (sin^2x + cos^2x) * (sinx + cosx) = sinx + cosx
sin^3 x (1+ctg x) + cos^3 x (1+tg x)= sin^3x + sin^2x * cosx + cos^3x + sinx * cos^2x = (sin^2x + cos^2x) * (sinx + cosx) = sinx + cosx