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)