#### Answer

a) for all (x,y) except the $x$-axis (y=0) and the y-axis (x=0)
b) for all (x,y)

#### Work Step by Step

a) We know that $\sin t$ is defined for all real numbers. Thus, for all $(x,y)$ and there must not be zero in the denominator. Thus, for all $(x,y)$ except $y=x=0$
b) The minimal value for $\cos x$ is $-1$. Thus the denominator will never be zero. Hence, all real numbers.