Question

if the nth term of an ap is 2n + 1 what is the sum of its first 3 terms..
plzz answer it..

Answer 1

an = 2n +1. If n= 1 , a1 = 2(1) +1= 3 If n = 2 , a2 = 2(2)+1 = 5. d= a2-a1 = 5 -3 =2. Therefore, Sn = 3/2(2×3 +(3-1)2). = 3/2(6+2×2). =3/2(6+4). =3/2×10. =15.

Answer 2

nth term = 2n + 1

let first term = a
& common difference = d

let n = 2

nth term = 2×2 +1
= 5
a + d = 5 ------1

let n = 3

nth term = 2×3+1
= 7

a+2d = 7 ------2

2-1

a+2d = 7
-a- d = -5
-------------
d = 2

by putting the value of d in eq 1
a = 5-2
a = 3


S3 = n\2 ( 2a + (n-1) d)
= 3\2 (2×3+ 2×2)
= 3\2 ×10
15


Hope it helps !!!