Could someone please explain...

I have some expressions, I dont know the different between using function 'subs' and calculating directly. My purpose is calculate nuy, by substitute the varience 'rch' into 'atan(f./(2*pi*x))' .

When I use 'subs' function, it results the error, division by zero.

But when I calculate directly, there are not error, and I obtained the result? Could anyone explain me about this?

>> syms x

>> nuy = atan(f./(2*pi*x));

>> rch = 0:0.001:0.3603;

>> subs(nuy,x,rch);

Error using symengine Division by zero.

Error in sym/subs>mupadsubs (line 140) G = mupadmex('symobj::fullsubs',F.s,X2,Y2);

Error in sym/subs (line 125) G = mupadsubs(F,X,Y);

nuy = atan(f./(2*pi*rch));

Answer 1

[Stephen23]

If you are using R2013A or newer then use atan2 instead:

>> syms x
>> nuy = atan2(f,2*pi*x);
>> rch = 0:0.001:0.3603;
>> subs(nuy,x,rch)

[duong mai]

Ah, my f equals to 0.02, it is not a function. that is ' 0.02/(2*pi*x) '. I mean why it does not happen error when i calculate directly?

[Stephen23]

>> f = 0.02;
>> syms x
>> nuy = atan(f./(2*pi*x));
>> rch = 0:0.001:0.3603;
>> subs(nuy,x,rch)
ans =
       1.5708       1.2664       1.0098        0.815      0.67216      0.56691      0.48776     etc

[duong mai]

There appears error. My screen does not look like yours. Why?

[Stephen23]

Apparently it depends on the version that you are using:

https://www.mathworks.com/matlabcentral/newsreader/view_thread/340190

One workaround is shown here:

http://stackoverflow.com/questions/25892135/symbolic-math-toolbox-hitting-divide-by-zero-error-when-it-used-to-evaluate-to-n