(2a^2+10a)/(a^2+4a+4)除（a+1-12/(a+2))
=[a(2a+10)/(a+2)^2][(a+2)/(a^2+3a-10)]
=[2a(a+5)/(a+2)^2]{(a+2)/[(a-2)(a+5)]} 十字相乘
=2a/(a^2-1)