B737200
Topic Author
Posts: 211
Joined: Sun Feb 20, 2005 5:30 pm

### Maths Problem

Hi,

I have a maths problem and since I've seen people asking all sorts of questions over here I thought "why not?"

So I need to solve this equation: 2.64 = (a) + ln(a)

where a is the variable i'm looking for and ln is the function ln [log_e].

The only thing I could come up with is to insert values of a and repeat the process correcting a as I go.

Anything better than iteration?

Thanks.

oly720man
Posts: 5809
Joined: Fri May 21, 2004 7:13 am

### RE: Maths Problem

Is this in any particular bit of maths?

One solution would be to use a series solution for ln(a) which would give a quadratic/cubic you could solve.

http://en.wikipedia.org/wiki/Natural...arithm#Derivative.2C_Taylor_series
wheat and dairy can screw up your brain

B737200
Topic Author
Posts: 211
Joined: Sun Feb 20, 2005 5:30 pm

### RE: Maths Problem

Its actually a fluids problem but I got stuck on the mathematical part.

The problem has water flowing out of a tank through an orifice whilst water is also flowing into the tank.

I ended up with an integral which I solved using a given result and I ended up with the above equation. It should be correct since my calculator worked it out and the answer matched but our lecturer is a bit fussy so I'm not sure if he'd except that in an exam.

I'll take a look at the link you sent, it seems somewhat familiar, maybe we did it last year in our maths unit. Hopefully something like that would be in the equation sheet we get, good luck trying to remember all the funny mathematical solutions to stuff that MIGHT come out. I get the feeling he is going to test us on mathematics as much us he is going to test us on our fluid's knowledge.

What nudged me towards the iteration method was the fact that the question states the we are to give an approximate answer; of course using a series solution would also yield and approximate value so maybe this is it.

Thanks, series totally slipped my mind, maybe its because I can't stand them

BMI727
Posts: 11300
Joined: Mon Feb 02, 2009 9:29 pm

### RE: Maths Problem

 Quoting B737200 (Reply 2):The problem has water flowing out of a tank through an orifice whilst water is also flowing into the tank

Perhaps looking at the differential equations for a mixing problem would be of help.
Why do Aerospace Engineering students have to turn things in on time?

Yellowstone
Posts: 2821
Joined: Wed Aug 16, 2006 3:32 am

### RE: Maths Problem

 Quoting B737200 (Reply 2):What nudged me towards the iteration method was the fact that the question states the we are to give an approximate answer; of course using a series solution would also yield and approximate value so maybe this is it.

What you want to use here is something called Newton's method.

http://en.wikipedia.org/wiki/Newton%27s_method

First, rewrite your equation so that it's equal to zero. f(a) = a + ln(a) - 2.64 = 0

Next, you pick some value of a, call it a_0, that is decently close to the right answer. Then, calculate the following, where f ' (a_0) is the derivative of the function f evaluated at a_0.

a_1 = a_0 - ( f(a_0) / f ' (a_0) )

Calculate a_2 in the same fashion, replacing a_0 with a_1. Repeat until a_n converges.
Hydrogen is an odorless, colorless gas which, given enough time, turns into people.

Fabo
Posts: 1154
Joined: Tue Aug 16, 2005 1:30 am

### RE: Maths Problem

Why repeat? If I understood right, you can use limit. You should get to about 1.96467 if I am not mistaken.

I will try figuring out how to isolate a, but do not expect much from me right now...
The light at the end of tunnel turn out to be a lighted sing saying NO EXIT

B737200
Topic Author
Posts: 211
Joined: Sun Feb 20, 2005 5:30 pm

### RE: Maths Problem

Firstly thanks for all the input, I wasn't expected this much help for my little mathematical conundrum.

 Quoting Yellowstone (Reply 4):What you want to use here is something called Newton's method.

I was actually studying it this week, when I was trying the fluids problem I hadn't really gone over my maths note yet. Seems to bring a bit of order to my "lets just throw in some values" method.

 Quoting Fabo (Reply 5):Why repeat? If I understood right, you can use limit. You should get to about 1.96467 if I am not mistaken

By using a limit you mean with Newton's method? The version we learned didn't use that, I'll look around but if I misunderstood you I'll be on a wild goose chase for nothing.

 Quoting Fabo (Reply 5):I will try figuring out how to isolate a, but do not expect much from me right now...

Don't worry if you're busy, I understand and thanks for your input.

 Quoting BMI727 (Reply 3):Perhaps looking at the differential equations for a mixing problem would be of help.

I derived a differential equation. The one I derived was:

Q_out - Q_in = A_tank * (dh/dt) where Qs are volume flow rates, A is area and h is the height of water level in the tank.

On working it out using a given result printed under the question itself I ended up with the 2.64 = lna +a.

Not sure if you are referring to the same thing.

Once again thanks a million, very helpful.

ag92
Posts: 1045
Joined: Wed Jul 26, 2006 5:23 pm

### RE: Maths Problem

I don't know if I will be of any help, but I would just graph it, and calculate the value through a graph

Flighty
Posts: 8896
Joined: Thu Apr 05, 2007 3:07 am

### RE: Maths Problem

Use my very most favoritest website ever:

http://www.wolframalpha.com/input/?i=2.64%3D+a+%2B+ln%28a%29

You're welcome.

Fabo
Posts: 1154
Joined: Tue Aug 16, 2005 1:30 am

### RE: Maths Problem

Flighty: That is what I used to come to 1.96467 I mentioned   Nice web, for sure.
The light at the end of tunnel turn out to be a lighted sing saying NO EXIT

waterpolodan
Posts: 1604
Joined: Sun Feb 13, 2005 3:46 pm

### RE: Maths Problem

Having Maths trouble? Might I suggest this video as a useful tool-

B737200
Topic Author
Posts: 211
Joined: Sun Feb 20, 2005 5:30 pm

### RE: Maths Problem

Thanks for the website, its now in my favorites.

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