х+у -(Х^3+y^3)=(x+y)(1-(x^2-xy+y^2)), т.к. х^3+y^3=(x+y)(x^2-xy+y^2)
x + y - x³ - y³ = x + y - (x³ + y³) = (x + y) - (x + y)( x² - хy+ y²)=
= (x + y)(1 - ( x² - хy+ y²)) = (x + y)(1 - x² + хy - y²) = (x + y)(1 - x² + хy - y²)
