4. Find the limit, if it exists, or show that the limit does not exist.\\
a) $\lim_{(x,y)\to(1,-1)} 2xy^2 + 4xy - y^3$\\
b) $\lim_{(x,y,z)\to(1,-1,2)} \ln(25 - x^2 - y^2) + \sqrt{z}$\\
c) $\lim_{(x,y)\to(0,0)} \frac{x^6 - y^6}{x^4 + x^2y^2 + y^4}$\\
d) $\lim_{(x,y)\to(0,0)} \frac{x^2 + y^2}{\sqrt{x^2 + y^2 + 1} - 1}$\\
e) $\lim_{(x,y)\to(2,2)} \frac{y^2 - 4}{xy - 2x}$\\
f) $\lim_{(x,y)\to(\frac{\pi}{2}, 0)} \frac{y \ln y \cos^2 x}{1 - \sin x}$\\
g) $\lim_{(x,y)\to(0,0)} \frac{2xy}{x^2 + 2y^2}$\\
h) $\lim_{(x,y)\to(0,0)} \frac{y^3 + x^3}{xy^2}$
