Let {an} be an arithmetic sequence with a tolerance of - 2 if a1 + A4 + A7 + +A97 = 50, then A3 + A6 + A9 + +A99 equals () A. 82B. -82C. 132D. -132

Let {an} be an arithmetic sequence with a tolerance of - 2 if a1 + A4 + A7 + +A97 = 50, then A3 + A6 + A9 + +A99 equals () A. 82B. -82C. 132D. -132

Because {an} is an arithmetic sequence with tolerance of - 2, A3 + A6 + A9 + + A99 = (a1 + 2D) + (A4 + 2D) + (A7 + 2D) + +(A97 + 2D) = a1 + A4 + A7 + + A97 + 33 × 2D = 50-132 = - 82

Let {an} be an arithmetic sequence with a tolerance of - 2 if a1 + A4 + A7 + +A97 = 50, then A3 + A6 + A9 + +A99 equals ()
A. 82B. -82C. 132D. -132

Because {an} is an arithmetic sequence with tolerance of - 2, A3 + A6 + A9 + + A99 = (a1 + 2D) + (A4 + 2D) + (A7 + 2D) + +(A97 + 2D) = a1 + A4 + A7 + + A97 + 33 × 2D = 50-132 = - 82

If S10 = S20, then S30=______ ．

∵ S10 = S20, ∵ 10A1 + 10 × 92d = 20a1 + 20 × 192D, ∵ 2A1 = - 29d. ∵ S30 = 30a1 + 10 × 292d = 15 × (- 29d) + 15 × 29d = 0