[12(x+y)的立方+3(x+y)(x-y)的平方]÷6(x+y)
=2(x+y)²+1/2（x-y）²
=2x²+4xy+y²+0.5x²-xy+0.5y²
=2.5x²+3xy+2.5y²
如果本题有什么不明白可以追问,