Help! several variables T^T

1.Find and sketch the domain of ....

a) f(x,y) = cosec(|x|+|y|) <<<i got |x|+|y|not equal to 0,pi,2pi,.... at the end but i dont know what the Df graph looks like -*-

b) f(x,y,z) = ln(16-(x^2)-(y^2)-(z^2))

2.Determine the set of points at which the function is continuous.

f(x,y)= x(sqrtx)(sqrty)/(x^2)+(y^2) if (x,y) not equal to (0,0)

= 0 if (x,y) = (0,0)

3.Find the partial derivatives of ....

a)Find fx and fy of f(x,y) = arctan(x-y)

b)Find df/dx and df/dy of f(x,y) = g(x/y)

it's my assignment and i can't do it T_T could someone plz help me....