an+an-1=3(an-1+an-2),an+an-1是首项为a2+a1=7,公比为3的等比数列的第n-1项,an+an-1=7*3^(n-2).(1)an-3an-1=-(an-1-3an-2),an-3an-1是首项为a2-3a1=-13,公比为-1的等比数列的第n-1项,an-3an-1=(-13)*(-1)^(n-2)=13*(-1)^(n-1).(2)(1)的3倍加(2)式,除以4,得an=[7*3^(n-1)+13*(-1)^(n-1)]/4.