15. Use the properties of continuity to determine the set of points at which each of the following functions is continuous. Justify your answers. a. The function f defined by f(x,y) b. The function g defined by g(x, y) = c. The function h defined by d. The function k defined by k(x, y) = zy x + y h(x, y) = { if (x,y) = (0,0) if (x, y) = (0,0) = I+2y z-y sin(z) 1+e {*** if (x, y) = (0,0) if (x, y) = (0,0)