X^3 + y^3 = (x+y)*(x^2 - x*y + y^2)
sin^3(x) + cos^3(x) = (sinx + cosx)*(sin^2(x) - sinx*cosx + cos^2(x))
sin^2(x) + cos^2(x) = 1 - основное тригонометрич.тождество
(sinx + cosx)*(1 - sinx*cosx)
(sinx + cosx)^2 = sin^2(x) + 2sinx*cosx + cos^2x = 1 + 2sinx*cosx = 1 + 2a
2sinx*cosx = 2a + 1 - 1, sinx*cosx = a
(sinx + cosx)*(1 - sinx*cosx) = a*(1 - a) = a - a^2