原式=a(a+1)/(a^2-1)+(3a+2)/(a^2-1)-(2a+1)/(a^2-1)
=(a^2+a+3a+2-2a-1)/(a^2-1)
=(a^2+2a+1)/(a+1)(a-1)
=(a+1)^2/(a+1)(a-1)
=(a+1)/(a-1)