Question

find the sum of first three terms of an ap whose nth term is 2n + 1

Answer 1

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

Putting n = 1

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

$\sf{a_1 = 2 + 1 = 3}$

Putting n = 2

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

$\sf{a_2 = 4 + 1 = 5}$

Putting n = 3

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

$\sf{a_3 = 6 + 1 = 7}$

Sum of first three terms –

= $\sf{a_1 + a_2 + a_3}$

= 3 + 5 + 7

= $<b>15</b>$

Answer 2

we have : nth term =2n+1 this implies a =2.1+1=3 , a2= 2.2+1 = 5 a3=2.3+1=7 so a(first term)=3 and d(common difference) =5-3=2 now according to question, S3=n/2[2a+(n-d) ] now S3 =3/2(6+4) and S3 will be 15.