Question

find the sum of 13 terms of an AP whose an=5-6n plz it's urgent

Answer 1

hope it helps....if it is correct. ..
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Answer 2

Hope u like my process
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Formula to be used
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A n = a +(n-1)d

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

Given :-
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An = 5 - 6n = - 1 - 6n +6

$\: \: \: \: \: = - 1 + (n - 1)\times-6$
So,..

a = - 1 and d = - 6

Now..
S 13 =
$\\ \frac{13}{2} (2 \times ( - 1) + (13 - 1)\times-6) \\ \\ = \frac{13}{2} ( - 2 - 72) \\ \\ = \frac{13}{2} \times- 74 = 13 \times -37 \\ \\ = -481$
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