原式=(a²+3)/(a+1)(a-1)-(a+1)²/(a+1)(a-1)+(a+1)(a-1)/(a+1)(a-1)=[(a²+3)-(a²+2a+1)+(a²-1)]/(a+1)(a-1)=(a²-2a+1)/(a+1)(a-1)=(a-1)²/(a+1)(a-1)=(a-1)/(a+1)