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11 October, 13:54

I just need someone to check my answer:

What is the sum of the arithmetic sequence 3, 9, 15 ..., if there are 24 terms?

a) 1452

b) 1728 (my answer)

c) 2028

d) 2268

+3
Answers (1)
  1. 11 October, 16:48
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    Sum is n (an+a1) / 2

    an=a1+d (n-1)

    common differnce is 6

    first term si 3

    an=3+6 (n-1)

    24th term

    a24=3+6 (24-1)

    a24=3+6 (23)

    a24=3+138

    a24=141

    so nth term is 141

    n=24

    24 (3+141) / 2=12 (144) = 1728

    answer is B
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