xy²-x

　　=x(y²-1)

　　=x(y+1)(y-1)

　　[(a²-b²)/(ab)]÷(1/a-1/b)

　　=[(a+b)(a-b)/(ab)]÷[-(a-b)/(ab)]

　　=[(a+b)(a-b)/(ab)]×[-ab/(a-b)]

　　=-a-b