Tìm giá trị MAX MIN
a) f(x,y) = $3(\frac{x^{2}}{y^{2}}+\frac{y^{2}}{x^{2}})- 8(\frac{x}{y}+\frac{y}{x}) (x,y\neq 0)$
b) f(x,y) = $\frac{x^{4}}{y^{4}} + \frac{y^{4}}{x^{4}} -2 (\frac{x^{2}}{y^{2}}+\frac{y^{2}}{x^{2}} ) + \frac{x}{y}+\frac{y}{x} (x,y\neq 0)$
c) y= $2^{sin^{2}x}+2^{1+cos^{2}x}$
d) y= $\frac{2\sqrt{1-x^{4}}+\sqrt{1+x^{2}}+\sqrt{1-x^{2}}+3}{\sqrt{1+x^{2}}+\sqrt{1-x^{2}}+1}$