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25-05-2016
Mathematics

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What is the sum of the first 45 terms of the following sequence? 20, 18, 16, . . .
a. 1,080
b. 1,420
c. 1,530
d. 1,980

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Аноним Аноним

26-05-2016

Sn = (a1+an)* n / 2
an = a1+d(n-1)

d = 18-20 = -2
a45 = a1+d(45-1) = 20 - 2*(45-1) = -68
S45 = (20-68)*45/2 = -1080

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