Question

Find the sum of the arithmetic sequence. 3, 5, 7, 9, ..., 21

Answer 1

a=3. d=2. n=21. Sn=n/2(2a+(n-1)d). Sn=21/2(2×3+(21-1)2)
21/2(6+(20)2) =21/2( 6+40).
=21/2(46)
=21×23
=483

Answer 2

Given,arithmetic sequence

3, 5, 7, 9, ....., 21

Here,

First term ,a = 3

Second term =5

Common difference = 5-3 =2

Last term , l = 21

nth term = a+ (n-1)d

21 = 3+(n-1)2

(n-1)2 = 18

n-1 = 6

n = 7

Sum of n terms = n/2 (a+l)

=7/2 (3+21)

=7/2 (24)

=84

I HOPE THIS WILL HELP YOU !