Ответы на вопрос:
2) a3+a8+a10+a16+a18+a23=126,найдите сумму первых 25 членов an
a3 = a1 + 2d, a8 = a1 + 7d, a10 = a1 + 9d, a16 = a1 + 15d, a18 = a1 + 17d, a23 = a1 + 22d
a1*6 + 22d + 17d + 15d + 9d + 7d + 2d = a1*6 + 40d + 17d + 15d = a1*6 + 55d + 17d = a1*6 + 72d = 126
a1 + 12d = 21, a13 = 21
a1 = a13 - 12d
a2 = a13 - 11d
a3 = a13 - 10d
a12 = a13 - 1d
a14 = a13 +1d
a23 = a13 +10d
a24 = a13 + 11d
a25 = a13 + 12d
s_25 = a1 + a2 + a3 + + a23 + a24 + a25 = 25*a13 -12d -11d -10d - -d + d + + 10d + 11d + 12d = 25*a13 = 25*21 = 525
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