Find an explicit formula for the arithmetic sequence 170,85,0,-85,...170,85,0,−85,...170 , comma, 85, comma, 0, comma, minus, 85, comma, point, point, point.

For the rule of 78, for a 12-month period, the last term in the sequence is 12 and the series sums to 78. for an 10 month period, the last term is and the series sum is . for a 15 month period, the last term is and the series sum is . for a 20 month period, the last term is and the series sum is