## Adam1994 3 years ago find the sum of the arithmetic sequence having the data given. a1 = 5, an = 3, n = 20

1. Chlorophyll

Do you know which formula to apply?

I have no idea, I missed a few days of class and I came back to a whole new lesson. Kinda just tryin to get caught up right now

3. Chlorophyll

S_n = n ( a_1 + a_n ) / 2

4. ngocthi0101

chloro right

5. Chlorophyll

Do you know how to plug in?

nuh uh :/

7. Chlorophyll

Try a1 = ?

8. ngocthi0101

that easy

9. ngocthi0101

i can prove formula

10. Chlorophyll

Just go step by step!

11. ngocthi0101

no, sum sequense have formula, not step by step

12. Chlorophyll

@Adam1994 pls let me know a1?

a1=5

14. Chlorophyll

a_n?

15. ngocthi0101

ok, i know a litle english

a_n = 3

17. Chlorophyll

Would you double check this problem, I believe it's wrong!

18. Chlorophyll

Since n = 20 => we're looking for S_20

19. Chlorophyll

S_20 = 20 ( a_1 + a_20 ) / 2

20. Chlorophyll

Anyway, this is the formula for arithmetic Sum, just memorize it for your own benefit!