2sn+a(n+1)=5a(n+1)/3
sn=a(n+1)/3
s(n+1)=a(n+2)/3
a(n+1)+a(n+2)=5a(n+1)
a(n+2)=4a(n+1)
a(n+1)=4an所以
an=4^n