y²-9（x+y）²
=y²-[3(x+y)]²
=[y-3(x+y)][y+3(x+y)]
=[y-3x-3y][y+3x+3y]
=(-3x-2y)(3x+4y)
=-(3x+2y)(3x+4y)第二步怎么来的y²-9（x+y）²=y²-[3(x+y)]²（3²=9）=[y-3(x+y)][y+3(x+y)]（平方差公式）=[y-3x-3y][y+3x+3y]=(-3x-2y)(3x+4y)=-(3x+2y)(3x+4y)能看明白吗？看明白了 谢谢