7. Find all stationary points of the function f(x,y) = -x^3 + 4xy - 2y^2 + 1.
8. Does the function f(x,y) = 2y sin(x) have any stationary points? If so, find one. If not, explain why not.
In Exercises 9-16, use the formula to find all stationary points:
9. f(x,y) = -xy
10. f(x,y) = sqrt(xy)
11. f(x,y) = 3x^2 + 2y^2
12. f(x,y) = xy - y - 2x + 2
13. f(x,y) = 3x^2 - 6xy + 2y^3
14. f(x,y) = x + y + 3x^2 - 3y^2 - 8
15. f(x,y) = x - xy - y
16. f(x,y) = x^3 - 6xy - 4y^3