Which ones are the true statements?

Find the local maximum and minimum values and saddle point(s) of the function. if you have three-dimensional graphing software, graph the function with a domain and viewpoint that reveal all the important aspects of the function 5. f(x, y) xy y + y 6. f(x, y)-xy 2x 2y x-y 7. f(x, y) x-y)1 - xy) 8. f(x, y)y(e- ) 9. f(x, y)-x y* + 2xy 10. f(x, y) 2 x* + 2x? - y? 11. f(x, у) — х* — зк + 3ку? 12. f(x, y)xy3 - 3x2 - 3y2 - 9x 13. f(x, y) — х* — 2х? + у? — зу 14. f(х, у) — у сos x 15. f(x, y)e cos y 16. f(x, y)xye-lw? +y2\/2_ 17. f(x, y)xy e° 18. f(x, y)(x2y')e 19. f(x, y)y2 2y cos x, -1 7 20. f(x, y)sin x sin y, -