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 May 9th, 2017, 05:25 AM #1 Newbie   Joined: Nov 2016 From: Aus Posts: 24 Thanks: 0 Solving equation in Matlab Hi All I'm newbie and do not have that experience in Matlab but really in urgent help with this question? Could you please help with solving this equation for X in Matlab? S*e=(C.^2)/E*X^b+f*C*(X)^(b+a) S,e, E are constants C,f,b, and a are known row vectors of 12 elements, so expect 12 value for X S=328, e=0.0033, E=204000 this code comes with errors C= [777.5 777.5 798.75 820 832.75 832.75 841.25 862.5 883.75 905 926.25 947.] f=[0.630796209 0.652441176 0.642583333 0.632803922 0.626973922 0.626973922 0.623102941 0.613480392 0.603936275 0.594470588 0.585083333 0.57577451] b=[-0.09 -0.09 -0.09 -0.09 -0.09 -0.156 -0.09 -0.09 -0.09 -0.09 -0.09 -0.09 ] a=[-0.56 -0.56 -0.56 -0.56 -0.56 -0.485 -0.56 -0.56 -0.56 -0.56 -0.56 -0.56 ] b=[-0.09 -0.09 -0.09 -0.09 -0.09 -0.156 -0.09 -0.09 -0.09 -0.09 -0.09 -0.09 ]; c=[-0.56 -0.56 -0.56 -0.56 -0.56 -0.485 -0.56 -0.56 -0.56 -0.56 -0.56 -0.56 ]; N=[1 12]; SF=[777.5 777.5 798.75 820 832.75 832.75 841.25 862.5 883.75 905 926.25 947.5 ]; ef=[0.630796209 0.652441176 0.642583333 0.632803922 0.626973922 0.626973922 0.623102941 0.613480392 0.603936275 0.594470588 0.585083333 0.57577451 ]; E=204000; SMx=328.4; de2=0.033; for k=1:12 eqn=(de2*SMx-((SF(k))^2/E)*((2*N(k))^b(k))-ef(k)*SF(k)*((2*N(k))^((b(k)+c(k))))== 0); vpasolve(eqn,N(k)) end; Thank you so much Last edited by Naz Ar; May 9th, 2017 at 06:11 AM.
 May 9th, 2017, 06:27 AM #2 Senior Member   Joined: Feb 2016 From: Australia Posts: 1,765 Thanks: 623 Math Focus: Yet to find out. Could you please write out what you are trying to do exactly, and what errors you are getting. This is a mess.
May 9th, 2017, 06:58 AM   #3
Newbie

Joined: Nov 2016
From: Aus

Posts: 24
Thanks: 0

Quote:
 Originally Posted by Joppy Could you please write out what you are trying to do exactly, and what errors you are getting. This is a mess.

I want to solve the equation in terms of different inputs and get the N value for each input

I do not how to define the vector of the output, and how to recall the output from the loop

The error is ((Index exceeds matrix dimensions.

Error in sym/subsref (line 776)
R_tilde = builtin('subsref',L_tilde,Idx);

Error in equ_tion (line 14)
eqn=(de2*SMx-((SF(k))^2/E)*((2*N(k))^b(k))-ef(k)*SF(k)*((2*N(k))^((b(k)+c(k))))== 0);
)))

the code is

((
b=[-0.09 -0.09 -0.09 -0.09 -0.09 -0.156 -0.09 -0.09 -0.09 -0.09 -0.09 -0.09
];
c=[-0.56 -0.56 -0.56 -0.56 -0.56 -0.485 -0.56 -0.56 -0.56 -0.56 -0.56 -0.56
];
syms N ;
SF=[777.5 777.5 798.75 820 832.75 832.75 841.25 862.5 883.75 905 926.25 947.5
];
ef=[0.630796209 0.652441176 0.642583333 0.632803922 0.626973922 0.626973922 0.623102941 0.613480392 0.603936275 0.594470588 0.585083333 0.57577451
];
E=204000;
SMx=328.4;
de2=0.033;
for k=1:12
eqn=(de2*SMx-((SF(k))^2/E)*((2*N(k))^b(k))-ef(k)*SF(k)*((2*N(k))^((b(k)+c(k))))== 0);
vpasolve(eqn,N(k))
end;
)))

May 9th, 2017, 07:17 AM   #4
Math Team

Joined: Oct 2011

Posts: 13,986
Thanks: 995

Quote:
 Originally Posted by Naz Ar S*e=(C.^2)/E*X^b+f*C*(X)^(b+a)
As Joppy says: what a mess!

Is that same as:
S*e = [C^2 / E] * [X^b] + [f*C*X^(b+a)] ?

Simplify it a bit:
u = S*e, v = f*C, w = b+a
to get:
u = [C^2 / E] * [X^b] + [v*X^w]

Anyhow, with X^b and X^(b+a),
can't be solved directly...
Agree Sir Joppy?

 May 13th, 2017, 08:47 PM #5 Newbie   Joined: Nov 2016 From: Aus Posts: 24 Thanks: 0 Please help with this problem. I tried to make it clearer '' syms N b, c, e, and s =row vectors of ten elements A=6; (scalar value) B=e*s; M=b+c; eqn=(A-(s^2)*((2*N)^b)-B*((2*N)^(M))== 0); for k=1:10 vpasolve(eqn,N(k)); end; '' how to solve N for all the vector elements (inputs) This loop does not work as error come as follows Error using ^ Inputs must be a scalar and a square matrix. To compute elementwise POWER, use POWER (.^) instead. (tried but did not work) Error in equ_tion (line 16) eqn=(A-(SF^2)*((2*N)^b)-B*((2*N)^(M))== 0); (cannot figure out what is the wrong here) Thanks
May 13th, 2017, 09:41 PM   #6
Senior Member

Joined: Feb 2016
From: Australia

Posts: 1,765
Thanks: 623

Math Focus: Yet to find out.
Quote:
 Originally Posted by Naz Ar Please help with this problem. I tried to make it clearer '' syms N b, c, e, and s =row vectors of ten elements A=6; (scalar value) B=e*s; M=b+c; eqn=(A-(s^2)*((2*N)^b)-B*((2*N)^(M))== 0); for k=1:10 vpasolve(eqn,N(k)); end; '' how to solve N for all the vector elements (inputs) This loop does not work as error come as follows Error using ^ Inputs must be a scalar and a square matrix. To compute elementwise POWER, use POWER (.^) instead. (tried but did not work) Error in equ_tion (line 16) eqn=(A-(SF^2)*((2*N)^b)-B*((2*N)^(M))== 0); (cannot figure out what is the wrong here) Thanks
Code:
syms N
b = randn(10,1)';
c = randn(10,1)';
e = randn(10,1)';
s = randn(10,1)';
A = 6; %(scalar value)
B = e.*s;
M = b + c;
f1 = A -(s.^2).*((2.*N).^b)
f2 = B.*((2.*N).^(M))
eqn=((f1 - f2) == 0);
I'm pretty sure you can't use vpasolve for this.

 May 15th, 2017, 09:16 PM #7 Newbie   Joined: Nov 2016 From: Aus Posts: 24 Thanks: 0 Thank you Joppy for your response and suggestion When applied your code it does not solve the equation. do you advise any function to solve this eqn please Regards

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