# How to use fimplicit with a symbolic function

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Kyle Donk on 10 Feb 2020
Edited: Matt J on 10 Feb 2020
I have a symbolic function r and am trying to plot it using fimplicit but I keep on getting an error in Matlab. I have tried switching up the symbolic r function, but it isn't working no matter what I try. Any suggestions?
This is a school assignment, so no complete answers.
syms x y R
r=(((x-2).^2)+((y-4).^2)==R.^2);
z=(x/2)+1;
[xc,yc]=solve(((x-2).^2)+(y-4).^2==R^2, (x/2)+1==y)
%(b)
subs(r,R,2)
hold on
fimplicit(((x-2).^2)+((y-4).^2)-R.^2)
fimplicit(z)
hold off

Matt J on 10 Feb 2020
Edited: Matt J on 10 Feb 2020
Using matlabFunction, you can derive a non-symbolic version of your circle equation expression from the symbolic version,
>> circ=(((x-2).^2)+((y-4).^2)-R.^2)
circ =
(x - 2)^2 + (y - 4)^2 - R^2
>> circ_nonsym=matlabFunction(subs(circ,R,2))
circ_nonsym =
function_handle with value:
@(x,y)(x-2.0).^2+(y-4.0).^2-4.0
The latter can be used with fimplicit
>> fimplicit(circ_nonsym); shg