Question

Find the sum of 20 terms of the
series -5.-2. +1. + 4 +.........20
terms
407
470
O O O O
O 427
O 724​

Answer 1

Series : -5, -2, 1, 4, ........ , 20

d1 = -2 - (-5) = -2 +5 = 3

d2 = 1 - (-2) = 1 +2 = 3

d3 = 4 - 1 = 3

here, d1 = d2 = d3

so the series forms an AP.

AP: -5, -2, 1, 4, ..... , 20

a = -5

d = 3

n = 20

$sum = \frac{n}{2} (2a + (n - 1)d$

$= \frac{20}{2} (2 \times ( - 5) + (19)3) \\ \\ = 10( - 10 + 57) \\ = 10 \times 47 \\ = 470$

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