# problem with partial derivative

fbeleznay shared this question 1 year ago

Hi,

In the attached GGB file I define a function a(x,y) to give the total distance of the point (x,y) from three points A,B and C.

I would like to find the partial derivatives of a(x,y) with respect to x and also with respect to y. Geogebra calculates these, but I do not understand the resulting b(x,y) and c(x,y).

Geogebra understands a(x,y) as a proper two-variable function, and can calculate the values, but it does not understand b(x,y) as a two-variable function, it for example gives b(0,0) undefined.

I guess, I can redefine a(x,y) differently, but I am not sure if this is a bug or not in the partial derivative algorithm.

1

This is interesting :)

I tried to calculate the partial derivatives in the CAS View, and everything works fine until I don't assign them the functional notation.

I mean, by typing Derivative(a,x) I get the correct answer immediately, but if I type e.g. g(x,y)=Derivative(a,x) I get a timeout error message from the CAS (Calculation took too long and was aborted).

This doesn't happen with easier functions. Everything works fine with e.g. a(x,y)=x^2y+xy^2

I'm looping this to the developers.

1

meantime try a(x, y) = sqrt((x - x(A))² + (y - y(A))²) + sqrt((x - x(B))² + (y - y(B))²) + sqrt((x - x(C))² + (y - y(C))²)

possibly there is a problem with abs() for distance of points