Q5 of 67 Page 74

Find the sum of the first 51 terms of an arithmetic progression in which second and third terms are respectively 14 and 18.

Common difference = 18 – 14 = 4
∴ a = a2 – d = 14 – 4 = 10
First term a = 10 and common difference d = 4


Since the sum of n terms is
Sn =


Where,


a = First term of AP
d = Common difference of AP
and no of terms is ‘n’



So sum of first 51 terms S51
= 25.5 × [ 20 + 50×4]
= 25.5 × [ 20 + 200]
= 25.5 × 220
= 5610


Hence, Sum of first 51 terms is 5610.


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