20th term?

Algebra Level 2

The first term of an arithmetic sequence is -8. Find the 20th term if the sum of the first 20 terms is 790.


The answer is 87.

Leonardo Fajardo by Leonardo Fajardo , 31, Philippines

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1 solution

Syed Baqir
Sep 19, 2015

S n = 1 2 ∗ 20 ( − 8 + L ) = 790 S_{n} = \frac {1}{2} * 20 ( -8+ L ) = 790

Where L is the last term:

n = 790 ∗ 2 20 + 8 = 79 + 8 = 87 n= \frac {790 * 2 }{20} + 8 = 79 + 8 \ = 87

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