(x²+2x)²-(2x+4)²
=(x²+2x+2x+4)(x²+2x-2x-4) 把看作x²+2x一个数,把看作2x+4一个数运用平方差公式.
=(x²+4x+4)(x²-4)
=(x+2)²(x+2)(x-2)
=(x+2)^3(x-2)