[tex]M.\frac{x-y}{x^3+y^3}=\frac{x^2-2xy+y^2}{x^2-xy+y^2}[/tex]
[tex]\Leftrightarrow M=\frac{x^2-2xy+y^2}{x^2-xy+y^2}:\frac{x-y}{x^3+y^3}[/tex]
[tex]\Leftrightarrow M=\frac{(x-y)^2.(x^3+y^3)}{(x-y)(x^2-xy+y^2)}[/tex]
[tex]\Leftrightarrow M=\frac{(x-y)^2.(x+y)(x^2-xy+y^2)}{(x-y)(x^2-xy+y^2)}[/tex]
[tex]\Leftrightarrow M=(x+y)(x-y)\Leftrightarrow M=x^2-y^2[/tex]