Question

Sum of the frist 14 term of an ap is 1505 and its first term is 10 find its 28th term

Answer 1

Answer:

Sum of n terms = Sn = n/2 [ 2a + (n-1) d ]

where a = first term d = common difference  

therefore, S14 = 14/2 [2*10 + (14-1) d ] = 1505

solve for d, we get, 215 = 20 + 13d

hence, d = 15.  

now, an = a + (n-1) d

therefore, a25 = 10 + (25-1) 15

a25 = 10 + 24 * 15 = 370.

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