即a²+2+a=15原式=(a-1)/(a+2)×(a+2)(a-2)/(a-1)²+1/(a+3)=(a-2)/(a-1)+1/(a+3)=(a²+a-6+a-1)/(a-1)(a+3)=(a²+2a-7)/(a²+2a-3)=(15-7)/(15-3)=2/3