a的平方-4a+3分之a平方-4 乘于 a平方+3a+2分之a-3
=(a²-4）/（a²-4a+3)×(a-3)/(a²+3a+2)
=(a-2)(a+2)(a-3)/[(a-1)(a-3)(a+1)(a+2)]
=(a-2)/(a²-1)