## raytiller1 2 years ago am i right? A store displays six computers on a shelf side–by–side. If the first computer is eight inches wide and each successive computer is four inches wider than the previous one, find the total width of the computers on the shelf. Part 1: Describe the sigma notation used in answering the question above. (2 points) Part 2: Show all your work and answer the question. (3 points)

1. raytiller1

8 ∑ 4 + 4n n=1 ∑ 4 + 4n = 4 ∑ 1 + n 4 * 8 + 4 ∑ n 32 + 4 [8 * 9 / 2] 32 + 40 72 inches

2. jim_thompson5910

no, the answer is not $\Large \sum_{n=1}^{8}4+4n$ for part 1 Part 2 depends on part 1, so that is also incorrect

3. raytiller1

ok can you help me then @jim_thompson5910

4. jim_thompson5910

How many computers are we talking about here?

5. raytiller1

6

6. jim_thompson5910

so you're not summing to 8 up at the top (of the sigma), you're summing to 6

7. raytiller1

thats the problem above says so i guess so

8. jim_thompson5910

so it should be $\Large \sum_{n = 1}^{6}4+4n$ for part 1

9. raytiller1

so is the first and 2nd part for part 2 right?

10. jim_thompson5910

first part is, but the second is not...and you'd have to use 6 instead of 8

11. raytiller1

∑ 4 + 4n = 4 ∑ 1 + n 4 * 6 + 4 ∑ n 24 + 4 [6 * 7 / 2] 24 + 25 49 inches

12. raytiller1

?

13. jim_thompson5910

4 [6 * 7 / 2] doesn't become 25

14. raytiller1

15. raytiller1

108 inches ?

16. jim_thompson5910

it is 108 inches

17. jim_thompson5910

108 is the total final answer for part 2

18. raytiller1

can you help me with another problem?

19. jim_thompson5910

sure, what is it

20. raytiller1

help me with the steps i have an answer just don't know how? geometric partial sum ∑[i=1,5,3(-4)^(i-1)].

21. raytiller1

nevermind i found it

22. jim_thompson5910

You're basically summing 3(-4)^(i-1) from i = 1 to i = 5 So add up 3(-4)^(i-1) five times like so 3(-4)^(i-1) 3(-4)^(i-1) 3(-4)^(i-1) 3(-4)^(i-1) 3(-4)^(i-1) Then replace the first i with 1, the second i with 2, the third with 3, etc etc like this 3(-4)^(1-1) 3(-4)^(2-1) 3(-4)^(3-1) 3(-4)^(4-1) 3(-4)^(5-1) Now evaluate

23. raytiller1

but i do need help with What is the 5th partial sum of ∑[i=1,∞,-1+5n]

24. raytiller1

the answer i got for the last problem was 11,170

25. jim_thompson5910

same thing, but you're now doing it with -1+5n

26. raytiller1

i got 60 ?

27. jim_thompson5910

write out -1+5n five times -1+5n -1+5n -1+5n -1+5n -1+5n then replace the first n with 1, the second with 2, etc etc to get -1+5(1) -1+5(2) -1+5(3) -1+5(4) -1+5(5)

28. jim_thompson5910

Evaluate each piece, then add them all up

29. raytiller1

70?

30. jim_thompson5910

yes, you are correct the answer is 70