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2014-10-05T14:02:35+03:00
7+14+21+......+777=7x(1+2+...+111)=7x111x112/2=43512
1 5 1
2014-10-05T14:05:12+03:00
=7(1+2+3+....+111 )
=7(111*112:2)
=7(111*56)
=7*6216
=43512
2 3 2