Simplish maths question

[KennyR]

My indice algebra is rusty, but I can't find my old maths books. And google isn't helping.

Basically, I want to solve C=(1-D)^T for T. (^T means to the power of T). I just can't remember what law to use to get that T down from there. Anyone?

[Karlos]

Laws of logs apply here don't they?

log (x^y) = y.log x

C=(1-D)^T

ln (C) = T ln (1-D)

T = ln(C)/ln(1-D)

I would think

[KennyR]

Aha, thanks Karlos! :-D I remember now.

It just went totally out of my head there, and nothing I could do would remind me...it's amazing how quickly you forget the things. :-(

[Karlos]

KennyR wrote:

it's amazing how quickly you forget the things. :-(

What things?

Who are you and what have you done to my cuppa?

[KennyR]

Karlos wrote:
What things?

Well, I really meant bizarre abstract rules and laws governing algebra, but since it's obviously the flavour of the month here in Amiga.org, my answer is: socks.

[Karlos]

KennyR wrote:

but since it's obviously the flavour of the month here in Amiga.org, my answer is: socks.

Ick. I hadn't realised people had stooped so low as to be chewing on the things now...

Still, can only be as bad as aloe vera ;-)

-edit-

Which reminds me, I was supposed to be making some tea :lol:

[Karlos]

So what are you graphing/solving anyway? Whenever I see T I instinctively think of absolute temperature...

-edit-

But I'm damned if I can think of anything that has a direct power dependency on temperature ;-)

[KennyR]

Oh, something incredibly trivial. I was trying to work what battery manufacturers call "shelf life". They say that the shelf life of batteries is how long they take to get to 85% of original charge, just sitting there and not being used.

T=time, in years
D=discharge (per year)
C=charge level (1 being full, 0 being empty)

I worked out that C=(1-D)^T, but got stuck trying to get it for T. But with your help:

T = ln(C)/ln(1-D)

Cheap alkaline or zinc chloride batteries, for instance, have a self-discharge rate of 4% per year, therefore:

T = ln(0.85)/ln(1-0.04)

= 3.98

So it works out as a shelf life of 4 years. Which is exactly what they are advertised as having.

I needed to get this for other kinds of battery. It may be pointless, but it beats chewing a sock, even if I do need the fibre.

[Karlos]

KennyR wrote:
Oh, something incredibly trivial. I was trying to work what battery manufacturers call "shelf life". They say that the shelf life of batteries is how long they take to get to 85% of original charge, just sitting there and not being used.

Ah, I see.

I needed to get this for other kinds of battery. It may be pointless, but it beats chewing a sock, even if I do need the fibre.

Combustarse say: "man who eat vindaloo has no need of fibre"

I think you'd get more semi decomposed protein and sugar and amines than you would fibre from chewing "The Sock" ;-)

[KennyR]

Karlos wrote:
I think you'd get more semi decomposed protein and sugar and amines than you would fibre from chewing "The Sock" ;-)

Especially bloodline's socks

[Karlos]

KennyR wrote:

Karlos wrote:
I think you'd get more semi decomposed protein and sugar and amines than you would fibre from chewing "The Sock" ;-)

Especially bloodline's socks

That *is* "The Sock" :-)

Hmm, now you have me wondering. Are there any other serial sox offenders here that we don't know about?

Name and shame! :lol:

[KennyR]

None must resign themselves to the dubious pleasures of forbidden laundry now we've armed ourselves with the knowledge of batteries. What woman could resist pick-up lines like "Wow, do you know that NiMH batteries, once fully charged, will self-discharge while unused to 85% of capacity in only SEVEN DAYS!?!"

[Karlos]

I can just hear the enthusiastic feminine chorus of "Get your coat, you've pulled!" :-D

-edit-

Something tells me the catchy title of this thread is keeping people at bay :-D

Either that or they've actually read it :lol:

[Doobrey]

Karlos wrote:
Something tells me the catchy title of this thread is keeping people at bay :-D

I only read it cos I thought it might have been about the guy that could get the 13th root of a 100 digit number in under 30 secs..
 Wonder if he`s any more successful in pulling the girlies with that trick, compared to the battery trivia chatup line?

I know what you mean about forgetting the basics of maths, the other day at work it took me a while to work out what 2.5% of 100g was !! standing there talking to myself.. "10% is 10g,half it so 5% is 5g, half it again so 2.5% is..DOH " :-x

[blobrana]

@kenny
ok you said...
T=time, in years
D=discharge (per year)
C=charge level (1 being full, 0 being empty)

I worked out that C=(1-D)^T, but got stuck trying to get it for T. But with your help:

T = ln(C)/ln(1-D)

Cheap alkaline or zinc chloride batteries, for instance, have a self-discharge rate of 4% per year, therefore:

T = ln(0.85)/ln(1-0.04)

= 3.98

So where did you get the value D from....

It`S a bit like saying D= 1 x (D x 1)

 :-)