an=(11...1)×10^n+(22...2)
=(11...1)(10^n+2)
=(1/9)(99...9)(10^n+2)
=(1/9)(10^n-1)(10^n+2)
=(1/9)(10^2n+10^n-2).
余下的就是等比数列求和了.自己算算吧.