Question

The sum of the first 20 terms of the sequence a sub n = 7n + 2 is _____.
Confused...
any explanation will be appreciated :)

Answer 1

What's the first term?

Answer 2

2 right?

Answer 3

yes

Answer 4

what's the 20th term?

Answer 5

oh wait, are you starting at n = 0 or n = 1?

Answer 6

is that 20*2?

Answer 7

see thats the confusing part idk if its 1 or 0

Answer 8

hmm maybe your book will tell you which one to start with because both are valid

Answer 9

i think its 0

Answer 10

Alright, let's go with that then

Answer 11

So if n = 0, then a0 = 2 is the first term

n = 19 is the 20th term (since we started at 0 and not 1)

Answer 12

I mean, you plug in n = 19 to find the 20th term

Answer 13

so 7*19?

Answer 14

and if it was 1 it would be 7*20?

Answer 15

7*19+2

Answer 16

oh ok and then thats the answer

Answer 17

no, you need to find the first and 20th terms to help get you to the answer

Answer 18

The formula is


Sum of first n terms = (n*(first term + nth term))/2

Answer 19

So

Sum of first 20 terms = (20*(first term + 20th term))/2

Answer 20

is the first term 2

Answer 21

yes, if you start at n = 0

Answer 22

so Sum of first 20 terms = (20*(2 + 20th term))/2

Answer 23

yes

Answer 24

so then (40+20th term)/2??

Answer 25

first off, what is the 20th term? that would be very helpful

Answer 26

remember, plug in n = 19 and evaluate

Answer 27

oh its135

Answer 28

(40+135)/2

Answer 29

so it's really


Sum of first 20 terms = (20*(2 + 135))/2

Answer 30

Sum of first 20 terms = (20*(2 + 135))/2

Sum of first 20 terms = (20*(137))/2

Sum of first 20 terms = (2740)/2

Sum of first 20 terms = 1370

This is of course assuming we start at n = 0

Answer 31

oh ok i see 1370

Answer 32

Thank you soo much @jim_thompson5910

Answer 33

yw