=(a+1)+(2-7a)/(a+2)除以(a^2-4)/(a^2+4a+4)
=〔(a+1)(a+2)+(2-7a)〕/(a+2)除以〔（a+2）（a-2）〕/(a+2)^2
=(a^2-4a+4)/(a+2)除以(a-2)/(a+2)
=(a-2)^2/(a+2)乘以（a+2)/(a-2)
=a-2