@ @¶ìÿa@óÝy‹¾a@ u@ @RRR an = a1 + (n − 1)d −20 = 50 + (n − 1) × − 1 2 −70 = − 1 n + 1 22 −70 − 1 = − 1 n 22 − 141 = − n 22 n = 141 S141 = 141 (50 + (−20)) = 141 × 30 = 141× 15 2 2 S141 = 2115 () . 171 . 1, 2, 3, 4,…