4(2x+y) -9(x-2y)
问题描述：
4(2x+y) -9(x-2y)
答
4(2x + y)²- 9(x - 2y)²
= [2(2x + y)]² - [3(x - 2y)]²
= [2(2x + y) + [3(x - 2y)][2(2x + y) - 3(x - 2y)]
=(7x - 4y)(x + 8y)