Answers

  • Brainly User
2016-02-22T01:28:33+08:00
S_{n} =  \frac{n}{2} [2a₁ + (n-1)(d)]

572 =  \frac{n}{2} [2 (5) + (n-1) (2)]

572 = n/2 (10 + 2n - 2)

572 = n/2 (2n + 8)

572 = n² + 4n

Re-write as quadratic equation:

    n² + 4n - 572 = 0

Solve by factoring:
   (n - 22) (n + 26) = 0

    n - 22 = 0          n + 26 = 0
    n = 22               n = -26

Choose the positive root, n= 22.

ANSWER:  22 terms of arithmetic series 5, 7, 9, ... must be added to get the sum of 572.

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