原式=(a+1)(a-3)/[(a-2)(a-3)]-(a-1)(a-4)/[(a-1)(a-2)]
=(a+1)/(a-2)-(a-4)/(a-2)
=[(a+1)-(a-4)]/(a-2)
=(a+1-a+4)/(a-2)
=5/(a-2)