The question is given from ncert Book of class 10th Chapter no. 5 Ex. 5.3 Q. 2 (ii). In the following question you have to find teh sum of the given A.P. Also answer the question briefly.

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Given:-

a = 34 , d = – 2 , l = 10

We know that,

Sn = n/2 ( a + l)

We have to find out n =?..

An = a + ( n – 1) d

10 = 34 + ( n- 1) (- 2)

10 = 34 – 2n + 2

10 = 36 – 2n

– 26 = – 2n

n = 13.

S13 = 13/2 ( 34 + 10)

S13 = 13/2 x 44

S13 = 13 x 22

S13 = 286.