Solve function of MATLAB learning Note 009

The solve function in MATLAB is mainly used to solve the analytic solution or exact solution of the linear equation Group. For the resulting result is a symbolic variable, the numerical solution of any number of digits can be obtained by VPA ().

There are four main syntax definitions for the solve function:

Solve (eq)
Solve (EQ, var)
Solve (eq1, EQ2, ..., eqn)
g = Solve (eq1, EQ2, ..., eqn, var1, Var2, ..., Varn)

EQ represents the equation, var stands for the variable.

But in my use, solve can not help me solve the problem:

My purpose: Known control object G0 and expected cut-off frequency WC, the frequency at any amplitude

This was used at the outset:

Wc=solve (' 20*log10 (ABS (G0 (WC)) =10*log10 (3) ', ' WC '),%g0 (WC) is a specific transfer of WC, 10*LOG10 (3) is a specific amplitude

The result of debugging is that the WC is an empty matrix, suggesting that the equation (20*log10 (G0 (WC)) =10*LOG10 (3)) is invalid. Then add this in front: sym WC;

It's still not working.

Then use this method:

For w=0.1:0.1:20
L=20*LOG10 (ABS (wn^2/((w*i) * ((I*W) ^2+2*zeta*w*wn*i+6^2)));
If l== (10*LOG10 (3)-0.001)
Wc=w;
End
End

L is the specific letter of communication. When debugging or not, WC is still empty. found that the IF SELECT statement is not running at all.

is not the precision of the W is small, cannot make if the condition (L==10*LOG10 (3)) satisfies.

Then I changed the W precision to 0.01,0.001.

Then I think it is not such accuracy can not meet the conditions, so I changed the conditions: l>= (10*LOG10 (3) -0.001) &&l<= (10*LOG10 (3) +0.001)

The end is still not ...

One more question: I tested ABS (), the modulo value function and the solve () function, and found that if it was solving the real number equation, OK, there was no problem; but if the complex equation is not:

>> Clear
>> syms x y
>> [X,y]=solve (' x^2+y^2=1 ', ' x ', ' y ')
 
x =
 
 -(1-z^2) ^ (1)
  -z^2) ^ (a)
 
 
y =
 
 Z
 z

>> Clear
>> syms x y
>> [X,y]=solve (' abs (x*i+y^2) =1 ', ' x ', ' y ')
 
x =
 
-exp (2*u*i) *i + Z^2*i
 
 
y =
 
Z

What is this for?