Total beginner needs some help solving an equation

Question

Erik 2012-2-24

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Hi all, I'm a complete beginner with Matlab, I decided to start learning a bit about it recently as I would like to use Matlab to write a code for some calculations I am doing.

I don't need you guys to send me codes or anything, just some hints to point me in the right direction would be very much appreciated!

So here's a link to the equation that I want to solve ---> http://dl.dropbox.com/u/23857814/Equation.JPG

Just to clarify the equation, 'Cn' and 'lamda' are in fact known values, or let's say they are variables that i can give specific values to so that in the equation only 'K' is unknown.

So my question is, how do I go about to solve for K in Matlab? Like I said, just some tips would be great :)

Thanks in advance.

Erik

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Answer 1

G A 2012-2-24

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https://ww2.mathworks.cn/matlabcentral/answers/30124-total-beginner-needs-some-help-solving-an-equation#answer_38591

doc solve

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Erik 2012-2-24

Hey GA

Thanks a bunch, I tried this 'solve' function, by re-writing the equation in the OP and Matlab seems to return me an answer, and it looks something like this ...

=====================================================================

>> solve ('(pi^4/(4*K))*((1+0.9973*K/pi)/(pi*K/0.9973)^2)*(0.5*pi*K/0.9973*(((1+0.9973*K/pi)/(1-pi*K/0.9973))+(1-pi*K/0.9973))+log(1-pi*K/0.9973))= 0.0689')

ans =

-3.097648011376843898931207281328

=====================================================================

However, I have a follow up question. Like I said in my OP, 'Cn' and 'lamda' are variables that I will be changing regularly, and in the above case, Cn=0.0689 and lamda=0.9973, so to prevent from manually adjusting the equation every time I want to change 'Cn' and 'lamda', I would like to write a simple code that looks something like this ...

=====================================================================

clear

format long

Cn = input('Please entre desired value of power coefficient: ');

while Cn <= 0

Cn = input('Power coefficient cannot be negative or zero, please entre an appropriate value: ');

end

lamda = input('Please entre desired value of advance ratio: ');

while lamda <= 0

Cn = input('Advance ratio cannot be negative or zero, please entre an appropriate value: ');

end

solve ('(pi^4/(4*k))*((1+lamda*k/pi)/(pi*k/lamda)^2)*(0.5*pi*k/lamda*(((1+lamda*k/pi)/(1-pi*k/lamda))+(1-pi*k/lamda))+log(1-pi*k/lamda))= Cn')

=====================================================================

This results in an error that looks like this ...

Warning: Explicit solution could not be found.

> In solve at 160

In calc at 15

ans =

[ empty sym ]

I still don't know Matlab well enough to understand what the error is suggesting and frankly I have no idea how to resolve this, any tips?

Thanks again, very much appreciated!

Erik

Sean de Wolski 2012-2-24

uses the SUBS function to substiture values into symbolic variables:

doc subs

G A 2012-2-24

Erik, you can try this way:

lamda=0.9; Cn=0.06;

syms k

solve (eval('(pi^4/(4*k))*((1+lamda*k/pi)/(pi*k/lamda)^2)* (0.5*pi*k/lamda*(((1+lamda*k/pi)/(1-pi*k/lamda))+(1-pi*k/lamda))+log(1-pi*k/lamda))-Cn'),k)

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