原式=[(a+1)/a(a-1)-4/(a+1)(a-1)]÷[(a+3)(a-1)/(a+3)]={[(a+1)²-4a]/a(a+1)(a-1)]}÷(a-1)=(a²+2a+1-4a)/[a(a+1)(a-1)²]=(a²-2a+1)/[a(a+1)(a-1)²]=(a-1)²/[a(a+1)(a-1)²]=1/(...