Question

the third term of an ap is 1 and 6th term is -11 .determine it's 15th term and Rth term

Answer 1

Solution :

The sum of first n terms of sequence,

Sⁿ = n(n+2)

Let n =1

S¹ = 1(1+2) = 1(3) =3 t¹=s¹ =3

t¹ = 3

Let n= 2,

s²= 2(2+2) = 2(4) = 8

S² = 8 .: tⁿ = sⁿ-sⁿ-¹

t² = 8–3 :. t² = 5

t² = 5

Let n=3,

S³ = 3(3+2) = 3(5) = 15 :. t³= s³-s²

t³ =15–8 = 7

t³ =7

:. The first three terms of given sequence is 3,5,7.

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