Q. 84.5( 181 Votes )

Find the sum of first 51 terms of an AP whose second and third terms are 14 and 18 respectively.

Answer :

To Find: S51

Given: a2 = 14, a3 = 18


We have; a2 = 14, a3 = 18 and n = 51


We know,


d = a3 – a2 = 18 – 14 = 4


Therefore, d = 4


a2 – a = 4


14 – a = 4


a = 10

So, first term = 10

Now, sum can be calculated as follows:


Where, a and d are first term and common difference respectively.


= 51 x (20 + 200)/2


= 51 x 110


= 5610

S51 = 5610

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