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