已知数列{an},a1=1,a2=2,an=an-1+an-2,(n≥3),则a6等于

问题描述:

已知数列{an},a1=1,a2=2,an=an-1+an-2,(n≥3),则a6等于

a6
=a5+a4
=a4+a3+a3+a2
=a3+a2+a2+a1+a2+a1+a2
=a2+a1+4a2+2a1
=5a2+3a1
=5*2+3
=13