Only questions 14 , 15

In Exercises 13-16, find and sketch the level curves f(x, y) = c on the same set of coordinate axes for the given values of c. We refer to these level curves as a contour map. 13. f(ry) = x + y - 1, c = -3. -2. -1, 0. 1, 2, 3 14. f(x. y) = x + y. c = 0. 1, 4. 9, 16, 25 15. f(x. y) = xy. c = -9. -4. -1, 0, 1, 4.9 16. f(x. y) = V 25 c = 0, 1, 2, 3, 4