公分母：3(a+1)^2(a+2)
a/(a^2+3a+2)=3a(a+1)/[3(a+1)^2(a+2)]
a/(a^2+2a+1)=3a(a+2)/[3(a+1)^2(a+2)]
-1/(3a+6)=-(a+1)^2/[3(a+1)^2(a+2)]