x^7+y^14=x^7+(y²)^7=
=(x+y²)[(x^6)-(x^5)y²+(x^4)(y²)²-x³(y²)^³+x²(y²)^4-x(y²)^5+(y²)^6]=
=(x+y²)[(x^6)-(x^5)y²+(x^4)y^4-x³y^6+x²y^8-xy^10+y^12]