4乘括号x加2y的平方减25乘括号x减y的平方= [2(x+2y)]² - [5(x-y)]²= [2x+4y]² - [5x-5y]²= (2x+4y+5x-5y)(2x+4y-5x+5y)= (7x-y)(-3x+9y)= -3(7x-y)(x-3y)
