Geometric sequence question gogogogo

[craptakular]

Find the number of terms in the following sequence:

5, -10, 20, ... , -40960

Now I have worked it out wrongly as follows:

r=-2
a1=5

an = a1 r^(n-1)
-40960 = 5 * -2^(n-1)

log 40960/log 2 = 15

-40960 = 5 * -2^(15-1) Which I have got wrong...

By testing the next number down, I get the right answer of 14.... but how do I do this properly and not guess after a **** up.

Where have I cocked up? lol

 

[Philtor]

This is not a Oxford uni Forum Oxy mate, however to answer your question..... Join all the dots together and I see a Giraffe! It's a trick question and the answer is Giraffe!! Ahh Haa !!

 

[SideWinder]

This is not a Oxford uni Forum Oxy mate, however to answer your question..... Join all the dots together and I see a Giraffe! It's a trick question and the answer is Giraffe!! Ahh Haa !!

There are a lot of very intelligent people who browse these forums...always maths threads popping up.

 

[craptakular]

I'm 28 and not sitting my exams, just trying to improve my maths.

 

[[FnG]magnolia]

OCUK - getting you through your homework because you're dumb and lazy

 

[craptakular]

lol this isn't homework I'm just trying to see if I can do "maths" before I waste ££££ on a masters

 

[Lightnix]

I did -40960 = 5*-2^(n-1)
-40960/5 = -2^(n-1)
log(40960/5)/log(2) = n-1
13 = n-1
n = 14

in short: You forgot to divide by 5. Just remember you're doing your algebra all cool and ****.

 

[Philtor]

Don't waste $$ on exams. Trust my answer.... It's a big number or animal. One of the two. Likely a big animal but then again im not sure.

 

[craptakular]

I did -40960 = 5*-2^(n-1)
-40960/5 = -2^(n-1)
log(40960/5)/log(2) = n-1
13 = n-1
n = 14

in short: You forgot to divide by 5. Just remember you're doing your algebra all cool and ****.

Thank you so much!!!

So this is how I would do all these types of questions? I forgot the algebra bits :/ lol I got the previous ones right by luck then lol

 

[Lightnix]

Thank you so much!!!

So this is how I would do all these types of questions? I forgot the algebra bits :/ lol I got the previous ones right by luck then lol

You're welcome mate.

Yes absolutely, just remember that you need to get that a term to the other side by dividing both sides by a before you convert it to a logarithm. Then when you rearrange the geometric sequence into log form equate the expression of the logarithm to the expression of the exponent (n-1) so you can see how to rearrange it to find n and you should be golden. It seems like you had the general idea down though, I'd just make sure to put emphasis on writing out each step of the problem down carefully.

 

[Philtor]

You're welcome mate.

Yes absolutely, just remember that you need to get that a term to the other side by dividing both sides by a before you convert it to a logarithm. Then when you rearrange the geometric sequence into log form equate the expression of the logarithm to the expression of the exponent (n-1) so you can see how to rearrange it to find n and you should be golden. It seems like you had the general idea down though, I'd just make sure to put emphasis on writing out each step of the problem down carefully.

(....) = Giraffe

(n-1) = Dangerous substance

Hence (....) + (n-1) = Elephangiraffe thing.

Sorry I will leave this thread alone now. I got a Masters in Elephants and look where it got me !!

 

[Lightnix]

(....) = Giraffe

(n-1) = Dangerous substance

Hence (....) + (n-1) = Elephangiraffe thing.

Sorry I will leave this thread alone now. I got a Masters in Elephants and look where it got me !!

Zoologists be damned.

Also we need more threads that have questions that approximate A-Level maths, that way I can pretend reading this forum counts as revision.

 

[craptakular]

expect more maths from me then lol.

when i did gcse like 15 yrs ago the internet hadn't reached suffolk lol.

i used to get so confused with text books i would cry, nowadays there are youtubes forums and all sorts explaining these maths terms so clearly. there actually maybe some hope i can overcome my crap maths and do this masters course i really wanna do. thanks again so much

 

[ojrules]

expect more maths from me then lol.

when i did gcse like 15 yrs ago the internet hadn't reached suffolk lol.

i used to get so confused with text books i would cry, nowadays there are youtubes forums and all sorts explaining these maths terms so clearly. there actually maybe some hope i can overcome my crap maths and do this masters course i really wanna do. thanks again so much

check out khan academy.. there are absoultely tonnes of videos for different subjects, i think it was around 5,000... should be worth a look- its aimed towards Alevel and lower, but i'm in my first year Meng course and there were some really useful videos- infact the notes on the videos were better than my stats lecturers notes...