jpw006
幼苗
共回答了7个问题 举报
(1) S15 = (A1+A15)×15/2 = (A3+A13)×15/2 = 225 ,
将A3 = 5带入,解得 A13 = 25 ,
A12 = A3+10d , d = 2 ,
所以 An = A3+(n-3)d = 2n-1
(2)A3+A8 = A6+A5 = 22 ,A6 = 7,
A5 = 15
(5)S10 , S20-S10 , S30-S20 成等差数列,
S10=10,S20-S10=20,S30-S20=30,
S30 = 60
1年前
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