x²-4y²+2x-4y
=x²+2x+1-(4y²+4y+1)
=(x+1)²-(2y+1)²
=(x+1+2y+1)(x+1-2y-1)
=(x+2y+2)(x-2y)(x+1+2y+1)(x+1-2y-1)这一步为啥把x+1看作一个数；把2y+1看作一个数；运用平方差公式：a²-b²=(a+b)(a-b)