(x+y+z)的二次方-(x-y-z)的二次方
=(x+y+z-x+y+z)(x+y+z+x-y-z)
=(2y+2z)*2x
=4x(y+z)为什么(2y+2z)*2x=4x(y+z)括号里面的提公因式2(2y+2z)*2x=2*（y+x）*2x=4x(y+z)乘号可以省