Answer :

Common difference = 18 – 14 = 4

∴ a = a_{2} – d = 14 – 4 = 10

First term a = 10 and common difference d = 4

Since the sum of n terms is

S_{n =}

Where,

a = First term of AP

d = Common difference of AP

and no of terms is ‘n’

_{}So sum of first 51 terms S_{51}

= 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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