Question

if nth term of an AP is 2n+ 1 then what is the sum of its first 3 terms

Answer 1

${a_n}$$\mathsf{= \ 2n + 1}$

$\mathsf{Putting \ n \ = \ 1}$

${a_1}$$\mathsf{= \ 2(1) + 1}$

${a_1}$$\mathsf{= \ 3}$

$\mathsf{Putting \ n \ = \ 2}$

${a_2}$$\mathsf{= \ 2(2) + 1}$

${a_2}$$\mathsf{= \ 5}$

$\mathsf{Putting \ n \ = \ 3}$

${a_3}$$\mathsf{= \ 2(3) + 1}$

${a_1}$$\mathsf{= \ 7}$






$\mathsf{Sum \ of \ first \ 3 \ terms \ -}$

$\mathsf{= \ 3 + 5 + 7}$

$\textbf{= 15}$

Answer 2

nth term of A.P an = 2n + 1


put n = 1 , 2 , 3


first term a = 2(1) + 1 = 3


second term = 2(2) + 1 = 5


third term = 2(3) + 1 = 7


difference = 5 - 3 = 2

number of terms n = 3


sum Sn = n/ 2 * ( 2a + ( n - 1)d ]

sum of first 3 terms


S3 = 3/ 2 * [ 2(3) + ( 3 - 1) 2 ]





S3 = 3 ×10 / 2 = 15.


sum of first three terms = 15

Answer: sum S3 = 15

second method:

as above , we get

first term = 3 , second term = 5 ,

third term = 7


sum = 3 + 5 + 7 = 15