1.

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

Answer»

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

\(S_{25} = \frac{25}{2}[2(a) + 24(d)]\)

= 25 [58] 

= 1450



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