a3=2·a2=72.
又∵=2·,n∈N+,∴为等比数列.
∴=·2n-1=2n,
∴an=n2·2n.
(2)证明:cn==,
∴c1+c2+c3+...+cn
=+++...+<+++·
=+·<+·=+
==<=,所以结论成立.