Question

I need someone to double check me please (:
Find the sum of the arithmetic sequence.

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

Answer 1

39?

Answer 2

@hartnn

Answer 3

@ganeshie8

Answer 4

doesn't look correcct

Answer 5

start by finding out the number of terms in the sequence

Answer 6

11

Answer 7

try again

Answer 8

10*

Answer 9

sorry miscounted

Answer 10

last term = 21 :

a + (n-1)d = 21

3 + (n-1)*2 = 21

(n-1)*2 = 18

n-1 = 9

n = 10

So there are 10 terms in the sequence

Answer 11

n = 10
a = 3
L = 21

use the formula :
Sum of arithmetic series whose first term = a, last term = L is :
\[\large \dfrac{n}{2}(a+L)\]

Answer 12

120 (: @ganeshie8