anonymous one year ago can someone please help me... I need to pass math. what is the sum of the series? A. -220 B. -110 C. -95 D. 110

1. anonymous

$\sum_{n-1}^{20} (5-1)$

2. anonymous

that 1 in the parenthesis is supposed to be an n

3. anonymous

I did the equation but when I add all the number up I get a weird number not one that i need.

4. anonymous

That's an arithmetic sequence because it's linear. You can use this formula |dw:1435279685117:dw|

5. anonymous

$\sum_{n=1}^{20} (5-n)$?

6. anonymous

I tried this but when I add the numbers I don't get the answer that i need... Could you help further on please.

7. anonymous

you can do @peachpi way or you can break it in two $\sum_{n=1}^{50}5-\sum_{n=1}^{20}n$

8. anonymous

oops $\sum_{n=1}^{20}5-\sum_{n=1}^{20}n$

9. anonymous

I did 5 -1 -1 5 - 2 - 1 5 - 3 - 1 and so on

10. anonymous

the first summation is $$20\times 5$$ and for the second one it is $\frac{20\times 21}{2}$

11. anonymous

use @peachpi formula, you do not want to add up twenty numbers

12. anonymous

okay I will try

13. anonymous

@peachpi can you write you formula out I dont really understand it

14. anonymous

your formula is 5 -n. The sum goes from 1 to 20, so plug them in. What do you get when you sub in 1 for n? What do you get when you sub in 20 for n?

15. anonymous

5 - 1 = 4 5 - 20 = -15

16. anonymous

right...

17. anonymous

|dw:1435280659076:dw|

18. anonymous

-11

19. anonymous

5*-11

20. anonymous

-55

21. anonymous

$\frac{20}{2}\neq 5$

22. anonymous

10

23. anonymous

@satellite73

24. anonymous

so then 10 * -11

25. anonymous

-110

26. anonymous

yes

27. anonymous

thank you so much.

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