Q. 315.0( 1 Vote )

# Find the sum of first 21 terms of the A.P. whose 2nd term is 8 and 4th term is 14.

Given: a2 = 8 and a4 = 14 and n = 21

We know that,

a2 = a + d = 8 …(i)

and a4 = a + 3d = 14 …(ii)

Solving the linear equations (i) and (ii), we get

a + d – a – 3d = 8 – 14

-2d = -6

d = 3

Putting the value of d in eq. (i), we get

a + 3 = 8

a = 8 – 3 = 5

Now, we have to find the sum of first 21 terms.

S21 = 21 [5 + 10 × (3)]

S21 = 21 [35]

S21 = 735

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