由an+1=3an+1得an+1+1/2=3an+1+1/2=3(an+1/2),即（an+1+1/2）/(an+1/2)=3.又a1+1/2=1+1/2=3/2.即数列{an+1/2}是以3/2为首项，3为公比的等比数列。所以an+1/2=3/2(1-3^n)/(1-3)=-3/4(1-3^n).则an=-3/4(1-3^n)-1/2
