Q. 265.0( 3 Votes )

Sum of the first

Answer :

Given: Sum of first 14 terms of an AP is 1505 and first term of the series (a) = 10

S14 = 1505 …(i)


Since,


, [, a = 10 and n = 14]


S14 = 7[20 + 13d] = 140 + 91d = 1505 (from equation (i)]


140 + 91d = 1505


91d = 1505 – 140 = 1365


d = 1365/91 = 15


We have a = 10 and d = 15 and we have to find a25.


a25 = a + 24d [, an = a + (n – 1)d]


a25 = 10 + 24×15 = 10 + 360 = 370


Thus, its 25th term is 370.


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