x+1/x(x-1) 乘上 “(x-1)²/x" =x^2-1/x^2 2. 4/a^2-1 乘上 a-1/6a =2/(3a²+3a) 3. x^2-y^2/x 乘上 -x^2/(x-y)^2=(x²+xy)/(y-x)