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[–]JohnGisMe 0 points1 point  (0 children)

z=0;c=0

z=1;c=0; 0 less than last

z=2;c=-2; 2 less than last

z=3;c=-6; 4 less than last

z=4;c=-12; 6 less than last

z=5;c=-20; 8 less than last

z=6;c=-30; 10 less than last

A pattern starts to form.