for (U sub n) Un=3n-1, list U1+U2+U3+....+U20 and find its sum
Un=3n−1nUn12253841151461772082392610291132123513381441154416471750185319562059
sum:
2+5+8+11+14...47+50+53+56+5959+56+53+50+47...14+11+8+5+2sum61+61+61+61+61...61+61+61+61+61sum=12∗20∗61=610
