Maybe....

(ninja-ed!!)

Last Q on a piece of homework is a decidedly evil differentiation question.

Normal differentiation I can do fine, where y = something in x, find dy/dx etc.

This one, I have x = something in y but still need to find dy/dx, and the normal to the curve at y = 0

Anyone know how it works when x and y are swapped in the question but not in the dy/dx bit, or whether I can just ignore that and do it the same as normal, or what the heck is going on...

and I guess I deserved the ninja-ing, what with leaving the topic open in General... and my frequent ninja-ing of others  

hehehe Couldn't help it.

Fair 'nough

Any idea about differentiation?

Whoa... this question will take serious recollection on my part. Haven't differentiated anything for about 7 years.

Any way you could put the equation up?

Difficult with the lack of superscript for the stuff to the power of other stuff, but I'll use ^ for that

x = e^y (2y^2 - 3y + 4)

Thinking about it, if you have x = something in y, and you need to find dy/dx, isn't the process to make the equation into y = something in x and then differentiate that?

Maybe

But the equation looks too evil to put it into x = stuff

Oh wait...

yeah

I could do the question if y and x were reversed, it's the fact that I'm still finding dy/dx that's confusing me...

No, there's a way to do it....

the x bit will take on log. No?

with e it would be ln, (log to base e) but yeah... maybe

Hang on, I'm gonna find a maths book and paper, and see if I can work this out. Enough guessing

If x = e^y (2y^2 - 3y + 4)

Then ln x = ln [e^y (2y^2 - 3y + 4)]

ln e^y = y, but I don't know what happens with the part in brackets when you start throwing ln's around

According to the internets, I can find dx/dy then take the inverse of that

Which makes sense, since inverting dx/dy would logically give dy/dx

That's true...

it won't be pretty but it could well work

which also would be the same as making the equation y=something in x and differenting that

ok, I be done

I probably should have googled around first, the answer was surprisingly easy to find... even though normally looking for things like "dy/dx" confuses google with it's punctuation-ignoring ways.

So that worked?

Seemed to 

Woo hoo!

You've reminded me how much I used to know, and how much I miss it now. Might actually revisit it... I can't believe I couldn't remember the simple stuff!

If I've re-ignited a love affair with differentiation (or other maths), then maybe the topic was all worth it after all....

hehe, everyone else is going to come in here and see this topic... all full of posts (28 and counting)

but it's all over already

hahahah

Yeah, I used to LOVE differentiation.

I liked manipulating the equations for some reason.... and it made more sense than integration which I mainly hated

I know what you mean... making the little numbers dance 

Always satisfying when something looks like the most horribly snarled-up equation in the history of forever, but then it ends up mostly cancelling itself out and boils down to something nice and simple.

hhehehe Yeah!!

Dance is the word I was just thinking of while reminiscing

And the precision of it all too... that i miss. everything has it's place, it's purpose.

Ok, I shaddup now

you definitely resparked my interest in math.

this was beyond where i ever was, but i used to love math.

i loved it until i had a teacher in college (the first time around) ruined it for me. the man could not have cared less about math, it was horrible. i had come from a small high school, where i had 1 math teacher for 3 years and he had instilled a serious love of math in me.

oh well, i'll be taking some math again in the fall (most likely) and hopefully, i will be reminded even more of my past love.

thanks, guys.

Sometimes I think there's someone out there reading my thoughts -

Link

lol!

Y = Mx + B

we were taught y = mx + c, none of this crazy B stuff 

Y-axis formula

where m is the slope of the line and b is the y-intercept, which is the y-coordinate of the point where the line crosses the y axis. This can be seen by letting x = 0, which immediately gives y = b

Formula Y = Mx + b

Yes.

y = mx + c works too, we just got taught with a different letter 

Cool

I did mathematics at university.  I was pretty good at it then and I must admit that I have held on pretty well to the algorythmic concepts. However, I can't add a column of numbers up and get the same answer twice without a spreadsheet...

LOL!

Those formulas in the box above I still remember

toa cah soh.

Now, what did we use it for.... *thinks*

we had to use it for finding the missing piece of a triangle.

that reminds me, is algebra as useless in real life as i think it is?

Lemme think....

Not completely useless, I don't think

Algebra, as in pure algebra, is quite useful as a concept.  The importance of remembering the the techniques is important for low-level software development but most of the time the development toolkits will do the number crunching for you.

I did have a project last year which I had to engage a maths PhD to help out with.  It was quite strange, the PhD was 30 and the software developer was 58, I'd have expected it to be the other way around.  However, all the mainstream developers these days are all into C#, .NET, C++, Java and so on, so finding someone who can write in non-compiled code means looking to the older generation.

hello super king

add me

thanks

Question on Statistics:

Ok, a bag has 5 marbles in it. One marble is red, Four marbles are blue.
Each time you take out a marble, you do not look at what colour it is. You do not return it to the bag.
At the moment, the chances of pulling out the red marble are 1 in 5.

What are the chances of pulling out the red marble, if, each time you take a marble, the colour of the marble stays unknown, and the number of the marbles in the bag is reduced by 1 each time.

Eg:
5 Marbles in bag: 1 in 5 chance. (marble is not seen or replaced)
4 Marbles in bag: ...??...chance (marble is not seen or replaced)
3 Marbles in bag: ...??...chance (marble is not seen or replaced)
etc...
etc...

Damn I hate these questions

It's no wonder I had to repeat Stats in uni, and yet barely passed it the second time

I consider myself extremely good at maths, but statistics, I never got it

no idea why..

It just didn't seem very logical to me - mathematics based on chance

If I remember correctly, you have to take into account the fact that there are less marbles in the bag + the chance that the red is already out.... or something along those lines

yeah and then multiply it by the divisor of the rocks on the moon.. blaaahhhh

http://en.wikipedia.org/wiki/Analysis_of_variance

So yeah, if anyone can answer that for me, I'd be really grateful

Won't help much, but that's the sub-part - variant stats

LOL - if it won't help, then why post it

and that was damn quick!! you have that in your bookmarks or somethin?!?!

No, I searched for it. I knew what to look for, just couldn't remember the term.

Well, I posted it as a starting point if you wanted to look into it more!

oohh.. picky picky

*walks away, shoulders slumped*

oh, come back!! I didn't mean it like that!

Fine! I'm gonna make it my life's mission to find you a formula

I posted this on Yahoo! Answers as well and have this response (which is sort of the answer I was expecting):

There is a hard way and an easy way to do this. You can use logic to see that regardless of what happens on previous draws, your chance is always going to be 1/5 to pick the red marble (up to the 5th draw) because you don't have any information about the previous draws.

This can also be done using conditional probability (Bayes' rule).

For example, with 4 marbles in the bag, you calculate the probability that you pick a red marble given that a red marble was picked on the first draw and the probability that you pick a red marble given that a blue marble was picked on the first draw and then weight these probabilities by the probabilities of picking a red on the first draw and blue on the first draw, respectively.

i.e. Pr (red with 4 marbles in bag) =
Pr (red with 4 marbles in bag | red on first draw)*Pr (red on first draw) +
Pr (red with 4 marbles in bag | blue on first draw)*Pr (blue on first draw)
= (0/4)*(1/5) + (1/4)*(4/5) = 1/5.

The same method can be applied to future draws from the bag.

I suppose it makes sense, right?

Ah yes... that looks familiar

Ok, I also found this which should help:

http://www.abwcorporation.com/AIMS-Ch-2.pdf

Thanks!

I'm not great at stats... I'm having problems even understanding what the question is.

So you take a marble from the bag and what... don't see it? Then put it aside so it's out of the equation, and you need to know the chance of picking the red one on each turn?

yep

On turn 1, chance of a red is 20%

To pick red on turn 2 you would need to pick blue on turn 1 (80%), then red (from a choice of 4, so 25%), 0.8 * 0.25 = 20%

To pick red on turn 3 you would need to pick blue on turn 1 (80%), then again on turn 2 (75%) then red on turn 3 (33%) = 20%

To pick red on turn 4 you would need to pick blue on turns 1 through 3 (80%, 75% and 66%) then red on turn 4 (50%) =  20%

To pick red on turn 5 would need to pick blue on all previous turns (80%, 75%, 66% and 50%) which we just saw produces 20%, then pick out the one remaining which will be red (choice of 1 from 1).

So yeah.. always 20%, when you don't know anything about the results of previous picks.

Probability I can do, other stats stuff... not so much 

what's that......up in the sky?

it's a bird.    it's a plane.      it's super king!

faster than a speeding bullet.....

able to solve probability stats in a single bound.....

heheheh I like it!

Although it's strange that you take the first step into account all the time?

Yeah? I didn't understand it either, but the working seems to make sense.

Ok, I read thru the pdf thing again... and I understand why you take the first step into account.

I guess the bit about the colour of the marble picked being unknown kinda threw me for a bit

42

hahahaha

Lies, damn lies and statistics.

I like that one...

ah yes - i forgot about your namesake Arthur.

Who's that?

the guy that woke up with a hangover only to discover that his house was scheduled to be demolished. Of course that is by the by as his world (earth) was also scheduled to be demolished. Luckily his best friend was in fact an alien. All you need is the guide and a towel.

Right.... when you said name sake, I thought you meant someone else

i never did finish that story Simon

you should. Douglas Adams is (was) a very clever man.

although on saying that, his books aren't everybodies cup of tea

Not for everybody, but then there's always the radio show, or the movie

i never finished the movie either

Gah!

what're we gonna do woth you!!

did you not finish the movie through choice or mitigating circumstances?

there is only one movie i have never finished - i remember it all too well - it was called The Pest and it had John Leguiziamo in it (not sure of the spelling and too lazy to look it up). I really really disliked that movie - i cannot bring to mind a worse waste of money and time to make crap like that.

all the books are awesome.

so clever and funny.......definitely worth it.

The original radio series (from the late 70's early 80's) is a pure gem.  They were broadcast before the book was written.  The TV series was OK but the recent film was awful.

My grandparents have what's either an earlier film, or the TV series on video 

The books are probably my media-form of choice, but I never heard the radio show so I guess that was kind of inevitable.

Recent film... had some decent moments, if it wasn't in the shadow of all the previous HHGttG stuff it might even seem good, but as an addition to what's gone before, sub-par.

They'll have the TV series on video.  You can also get all the radio stuff on CD.

ah...'a trilogy in five parts'.......an appropriate topic for this, yes?

when i recently decided to read the entire series....i found a "complete version", which had been put together before douglas adams died.......it included a really nice preface where he discusses the whole history and makes a sort of time-line......and also a related short story.......

i thoroughly enjoyed it.

i agree with sk that the movie would have been better if it weren't for all that was already out there.