[(a+b)²（a-b)负二次方/2（a+b)负二次方（a-b)²]负三次方
=[(a+b)²(a+b)²(a-b)²/2(a-b)²]^(-3)
=1/[(1/2)(a+b)^4]^3
=(1/8)(a+b)^12