就是利用平方差公式啊.x^2-y^2=(x+y)*(x-y)
(1/a+1/b)²÷(1/a²-1/b²)
=(1/a+1/b)²÷[(1/a+1/b)*(1/a-1/b)]
=(1/a+1/b)÷(1/a-1/b)
=[(a+b)/ab]÷[(a-b)/ab]
=(a+b)/(a-b)