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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
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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