（x+y+z）5次方-x5次方-y5次方-z5次方 =[（x+y+z)^5-x^5]-(y^5+z^5) =（x+y+z-x）[(x+y+z)^4+x(x+y+z)^3+ x^2(x+y+z)^2+x^3(x+y+z)+x^4]- (y+z)(y^4-y^3z+y^2z^2-yz^3+z^4) =(y+z)[(x+y+z)^4+x(x+y+z)^3+x^2(x+y+z)^...