Question

second and fourth term of an A. P. is 12 and 20 respectively. find the sum of first 25 terms of that A. P. ​

Answer 1

T4 = a+3d = 13 …(1)

T10 = a+9d = 25 …(2)

Subtract (1) from (2)

a+9d-a-3d = 6d = 25–13 = 12

So d = 2.

From (1), a = 13 - 6 = 7

The first term is 7, the common difference is 2 and the 17th term = a+16d = 7+32 = 39.

Answer 2

This may be correct answer