In Exercises 3-20, identify and determine the nature of the critical points of the given functions.
3. f(x,y) = 2xy - 2x2 - 5y² + 4y - 3
4. f (x,y) = ln(x² + y² + 1)
5. f(x,y) = x² + y³ - 6xy + 3x + 6y
6. f(x,y) = y⁴ - 2xy² + x³ - x
7. f(x, y) = xy + $\frac{8}{x}$ + $\frac{1}{y}$
8. f(x, y) = $e^x$ siny
9. f(x, y) = $e^{-y}$ (x² - y²)
10. f(x, y) = (x + y)(1 - xy)
11. f(x, y) = x² - y³ - x²y + y