error when calculate differentiation of circle function

Hi:

I have a circle function: (x-x0)^2+(y-y0)^2=r^2;

in order to calculate th tangent of this function, I organized the function to y=sqrt(r^2-(x-x0)^2)+y0, and tried to calculate it in Matlab using "diff" function, however, Matlab reported error, below is the code, could anyone help me with this?

funcCircle = @(x)(sqrt(r^2-(x-x0)^2)+y0);
funcCircle_deri=diff(funcCircle,x);

Thanks!

Yu