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18.03.2013

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21. 1, 2, . .

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ai - c d, bi - , q.

S3 = (2a1 + 2d) 3 / 2 = 21 a1 + d = 7.

a1 - 1, a1 + d - 1, a1 + 2d + 2 - . :

(a1 + d - 1)2 = (a1 + 2d + 2)(a1 - 1).

a1 = 7 - d

d2 + 3d - 18 = 0, .. d1 = 3, d2 = -6.

d1 = 3 (.. ). a1 = 4. b1 = a1 - 1 = 3. b2 = a1 + d - 1 = 6, q = 2.

, :

S8 = [b1(q8 - 1)] / (q - 1) = 765.

: S8 = 765.


, , 2, 14/9. .

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, , a, a + d, a + 2d.

2, .. 3a + 3d = 2, a = 2/3 - d.

a2 + (a + d)2 + (a + 2d)2 = 14/9.

27a2 + 45d2 + 54ad = 14.

a = 2/3 - d, :

d2 = 1/9.

d = 1/3.

, . d = 1/3 1/3, 2/3, 1.

: 1/3, 2/3, 1.


, , 9, 18.

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, , b, bq, bq2, bq3.

:

1) bq2 = b + 9.

2) bq = bq3 + 18.

q :

9q + 18 = 0.

q = -2. b. b = 3.

: 3, -6, 12, -24.

: 3, -6, 12, -24.


, 7.

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, 7. 105 994 . a1 = 105, am = 994.

m, .. , 7. :

994 = 105 + 7(m - 1).

m = 128.

m S128 = (105 + 994) 128 / 2 = 70336.

: 70336.


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