In Exercises 12-15, use the contour diagram for f(x, y) in Figure 12.100 to suggest an estimate for the limit, or ex- plain why it may not exist. 0 2 4 6_ 10 - 10 - X 2 B 6 6 Figure 12.100 12. lim f(x, y) 13. lim f (x, y) (x,))-(2,1) (x,))-(-12) 14. lim f(x. y) 15. lim f (x, y) (x,))-(-20) (x, ))-+(0,0)