Choosen As Best Answer

15^2 = 15的平方

a15 = S15 - S14
S15 = -2 × 15^2 + 15 = -435
S14 = -2 × 14^2 + 14 = -378
a15 = S15 - S14 = (-435) - (-378) = -57 #

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首項 = a、公差 = d
a + (a+2d) + (a+4d) + ... + (a+28d) = -1980 ...(1)
(a+d) + (a+3d) + (a+5d) + ... + (a+29d) = -2190 ...(2)
(2) - (1)
d + d + d + ... + d = -210
15d = -210, d = -14

(a + a+28d)×15÷2 = -1980
2a - 392 = -264
2a = 128, a = 64 #

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