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2014-11-16T12:25:39+02:00
N = 2015 +2·2014·2015/2 = 2015 +2014·2015 = 2015(1+1014) = 2015² =p.p.
1+2+3+........+2014 = (n-2015) / 2 = (2015²-2015)/2 =2015·2014/2 = 2015·1007
sau direct : 1+2+3+......+2014 = 2014·2015/2 = 1007·2015