Question

A partial sum of an arithmetic sequence is given. Find the sum.
1+5+9+...+401

Answer 1

a=1, d=4
Sn=(n/2)(a1+a2)

I'm not sure how to plug in these numbers though!

Answer 2

@jim_thompson5910 :) do you have time?

Answer 3

it should be Sn = (n/2)(a1 + an)

Answer 4

an = a1 + d(n-1)

Answer 5

right right.

Answer 6

First you need to find n

Answer 7

an = a1 + d(n-1)

401 = 1 + 4(n-1)

n = ??

Answer 8

101 ?

Answer 9

yes

Answer 10

a1 = 1
an = 401
n = 101

Sn = (n/2)*(a1 + an)

Answer 11

er so
sn=20301

Answer 12

I got the same

Answer 13

wolfram agrees

http://www.wolframalpha.com/input/?i=1%2B5%2B9%2B...%2B401

Answer 14

haha yay! the wolfram stamp of approval x) thanks so much.

Answer 15

np