Question

Find the sum of the first 25 terms of an A.P. whose second and third terms are 14 and 18 respectively.

Answer 1

a2=14, a3=18. d=a3-a2=4.

Answer 2

Sum = 1450

Step-by-step explanation:

(i) Given: First 25 terms of an A.P. whose second and third terms are 14 and 18 respectively.

Find: Sum

Solution:

  n = 25

  Sum of AP = n/2 (2a + (n-1)d)

a2 = a + d = 14 ---------(1)

a3 = a + 2d = 18 ---------(2)

Therefore d = 18 -14 = 4

(1)(-2) + (2), we get: a = 10

So Sum = 25/2 (2(10) + 24(4))

       = 25/2 (20 + 96)

       = 25 * 58

        = 1450