The nth term of an A.P is given by (−4n + 15), Find the sum of first 20 terms of this A.P.

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Solution

$a_{n} = - 4 n + 15 \Rightarrow a_{1} = - 4 + 15 = 11 Also, a_{2} = - 8 + 15 = 7 Common difference, d = a_{2} - a_{1} = 7 - 11 = - 4 Now, S_{20} = \frac{20}{2} [2 \times 11 + (20 - 1) (- 4)] = 10 (22 - 76) = - 540$

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