Find the sum of the first 25 terms of an A.P. whose second and third terms are 14 and 18 respectively.
a2= 14
a + d = 14 (i)
a3= 18
a + 2d = 18 (ii)
Subtracting (i) from (ii), we get
d = 4
Putting the value of d in (i), we get
a = 14 – 4 = 10
S25= 
[2(a) + 24(d)]
= 25 [58]
= 1450
28