12(x+y)的平方-3(x-y)的平方
问题描述：
12(x+y)的平方-3(x-y)的平方
答
12(x+y)²-3(x-y)²
=3[4(x+y)²-(x-y)²]
=3[2(x+y)+(x-y)][2(x+y)-(x-y)]
=3(3x+y)(x+3y)