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21 April, 10:15

Find the sum of the first 21 terms of the sequence 5, 9, 13, 17, 21, ... a. 945 c. 947 b. 944 d. 946

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  1. 21 April, 10:32
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    Answer:A 945 is correct
  2. 21 April, 10:33
    0
    See how the arithmetic sequence is made:

    5, 5+4=9, 9+4=13, 13+4=17, ...

    So the 21th term will be:

    5 (first term) + 20 (n-1 term) •4 = 85

    And the sum of the first 21 is:

    21 (5+85) / 2 = 21•45 = 945
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