>>76
ヒント：
x^2 - y^2 = (x-y)(x+y)
x^3 - y^3 = (x-y)(x^2 + xy + y^2) = (x-y){(x+y)^2 - xyi}
x^4 - y^4 = (x-y)(x+y)(x^2 + y^2) = (x-y)(x+y){(x+y)^2 - xy}