Question

Find the sum of the first 20 terms: 1+3+5+7...

Answer 1

\[S_n = \frac n2(2a_1 + (n - 1)d)\]

Answer 2

is that a formula?

Answer 3

a = 1 here and d = 3 - 1 = 2

n = 20 here:

\[t_{20} = 1 + (20-1)(2) \implies t_{20} = 39\]

Now use:

\[Sum = \frac{n}{2}(First + Last)\]

n = 20 here..

Answer 4

Yes. That is the formula if you don't have \(a_n\).

Answer 5

yea, n = no of terms = 20

a = first term = 1

d = common difference = 2

Answer 6

First is 1 and Last is 39

n = 20 there..

Plug in the values and find the sum..

Answer 7

ok thx guys I will work it out myself and tell you what I got then u can check it for me.

Answer 8

\[s _{20}=400\]

Answer 9

Yup :)

Answer 10

ok thx

Answer 11

np :)