Question

Find the sum

4+4(.5)+4(.5)^2+....+4(.5)^12

Sn=a1(1-r^n)/1-r
n=13
a1=4

What's r?

Answer 1

@agent0smith

Answer 2

They make it easy by telling you r... which number is being multiplied each time?

Answer 3

.5?

Answer 4

Excellent :D now use the sum formula again

Answer 5

really thats r?!?! awesome!!!

Answer 6

wow this one is way way (way) easier than the previous integrals, isn't it?

Answer 7

lol yes, maybe she's meant to be doing them via integration, but meh

Answer 8

nah this is easier! lol

Answer 9

i hope not, but in any case this is a lot easier than finding the area in some sort of clover leaf using polar coordinates, or horrific partial fractions

Answer 10

so i got 7.999 and it says its wrong:(

Answer 11

(4(1-(.5)^13))/(1-(.5))

Answer 12

i get the same thing

Answer 13

the question says whats the value of the sum

Answer 14

and yeah i double checked the question and its written out right

Answer 15

http://www.wolframalpha.com/input/?i=4+sum+n+%3D0+to+12+%28.5^n%29

Answer 16

what am i doing wrong lol its probably something small!

Answer 17

does it say "give whatevever decimal places " or "round to the nearest something"?

Answer 18

Hmmm :/

Answer 19

nope:/

Answer 20

then stick with your answer, or write \(7.99902\)

Answer 21

its online homework:/

Answer 22

would i use a different equation and add 1 to the exponent? there was a formula my professor used and we did that but i have no idea what it was or why he was doing it lol....i really wish he spoke better english lol

Answer 23

I'm going to add them up one by one, on wolfram alpha.

Answer 24

do what u need to do:) lol

Answer 25

put this number in and tell her to shove it

http://www.wolframalpha.com/input/?i=4%2B4%28.5%29%2B4%28.5%29%5E2%2B4%28.5%29%5E3%2B4%28.5%29%5E4%2B4%28.5%29%5E5%2B4%28.5%29%5E6%2B4%28.5%29%5E7%2B4%28.5%29%5E8%2B4%28.5%29%5E9%2B4%28.5%29%5E10%2B4%28.5%29%5E11%2B4%28.5%29%5E12+%3D+

Answer 26

i added, check link above

Answer 27

Yes but i meant type out each term :P

Answer 28

you have more patience than i do

Answer 29

oooh
unless the question is really "what is r"? as written, in which case the answer is \(.5\) !!

Answer 30

copy and pasted +4(.5)^2 and edited the exponent a bunch of times, was quick

Answer 31

is it one of those on line "step by step" questions, where they walk you through it? maybe the question only wants you to identify \(r\)
after all, they have given you everything else, formula included

Answer 32

nope:/

Answer 33

@satellite73 I think Megan in all her genius is the one who gave everything after:
"Find the sum

4+4(.5)+4(.5)^2+....+4(.5)^12"

Answer 34

its not .5 just btw i tried it:/

Answer 35

ok then moveon.org you will not find another answer

Answer 36

Email your teacher @megannicole51 and tell her it's wrong (the online answer). If two guys with maths degrees (i'm assuming) and one university student can't figure it out, then it's obviously wrong ;)

Answer 37

he doesnt check his email until early mornings and this homework is due at 6am so its hopeless:/

Answer 38

hes a crazy old man bt hes so funny...i just wish he would check his email more

Answer 39

lol well yeah crazy old people don't be stayin on the innernet at night-time.

Answer 40

okay so the problem under it is....

4(.5)^3+4(.5)^4+4(.5)^5+.....+4(.5)^11

so n=9
a1=4(.5)^3
so then what is r for this one?

lets see if i can figure this one out and if we get this one wrong too then ill email my professor

Answer 41

explain how \(n=9\)?

Answer 42

But this time it doesn't start at n=1. But, we can just subtract those terms from the sum...

Answer 43

http://www.wolframalpha.com/input/?i=4%28.5%29^3%2B4%28.5%29^4%2B4%28.5%29^5%2B4%28.5%29^6%2B4%28.5%29^7%2B4%28.5%29^8%2B4%28.5%29^9%2B4%28.5%29^10%2B4%28.5%29^11

Answer 44

with props to @agent0smith

Answer 45

bc the square roots start at 3 and u just kinda count back...sorry thats a bad explanation lol

Answer 46

*square root
lol that was super terrible grammar

Answer 47

lol, they are exponents, not roots, square or otherwise

Answer 48

hahahahahah that just shows how tired i am...thank you

Answer 49

"super terrible grammar" is probably super terrible grammar ;)

Answer 50

thats why it was funny:) lol

Answer 51

:D

Answer 52

so any who.....how do we find r lol

Answer 53

4(.5)^3+4(.5)^4+4(.5)^5+.....+4(.5)^11

r looks the same as the last one

Answer 54

In a geometric series, 'r' is the common ratio and can be found by dividing any term by its previous term.

Answer 55

any 2 terms?

Answer 56

4(.5)^4 / 4(.5)^3 = 0.5 = r
4(.5)^5 / 4(.5)^4 = 0.5 = r

Answer 57

If you look at the series, the second term is the first term times .5
The third term is the second term times .5
So .5 is the common ratio.

Answer 58

awesome thank you:)

Answer 59

u r welcome!

Answer 60

okay so i got that second problem right! but i still cant figure out the first one!

Answer 61

Into Google I typed exactly "4(1 - .5^13)/(1 - .5) = " without the quotes and I get
7.9990234375.

Answer 62

@satellite73 hey so for the first problem i guess i just didnt put enough decimal points or something because i entered 7.9990234375 and it says it was correct! technology what are you guna do lol!

Answer 63

thank you i already got it:)

Answer 64

Haha awesome megan :)

Answer 65

thank you:D