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2014-06-01T13:18:22+03:00
2014-06-01T13:19:26+03:00
1+(1+1)+(1+1+1)+...+(1+1+1+1+1+1+1)=

=1+2+3+....+7=

=(1+7)/2*7=

=8/2*7=

=4*7=28