a/(a^2+3a+2）＝a/(a+1)(a+2)=a(a+1)/(a+1)^2(a+2)
a/(a^2+2a+1)=a/(a+1)^2=a(a+2)/(a+1)^2(a+2)
-1/(3a+6)=-1/3(a+2) =-1/3×(a+1)^2/(a+1)^2(a+2)