desmos:

2d: equation vs parametric equation: https://www.desmos.com/calculator/b59o6wnhgg

explanation for directional derivative:

https://www.desmos.com/calculator/6rswcscn3p

3d graph for f(x,y): https://www.desmos.com/calculator/f9doadjhva

3d graph of a curve:  https://www.desmos.com/calculator/ya3jsm9do5

3d graph(2) (x^2+3y^2)/(5x^2+2y^2)

https://www.desmos.com/calculator/wkvqgosgny

3d graph (3) (x^2 y)/(x^4+y^2)

https://www.desmos.com/calculator/i8begalhyo

3d graph (4) (x/y)

https://www.desmos.com/calculator/sf5am8opee

3d graph(5) abs

https://www.desmos.com/calculator/jdr8v5chaz