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2014-01-18T22:47:51+02:00
1=3*0+1;
4=3*1+1;
7=3*2+1;
.............;
n=3*x+1;
1+4+7+...+n-3=3*0+1 +3*1+1 +3*2+1 +...+3*x+1-3=3*(0+1+2+...+x-1)+1+1+1+...+1(x+1 nr)
=3*[(x*(x+1)/2)-1]+x+1
1 5 1