Mathematics - Calculus - Multivariable - Partial derivatives - Slicing a Surface

Show a 3D surface plot of f(x,y)=x²+y² (a paraboloid bowl). The student can 'slice' the surface by holding y constant (e.g., y=2) to see a 2D parabola in the x-direction. Or hold x constant to see the y-direction slice. The slope of each slice IS the partial derivative. ∂f/∂x = slope when moving in