5.f_x=2x,2=0
f_y=2y,3=0
Min at 36/13, max val not present
7.f_x=x,8x
f_y=y,18y
max at 8/3, min at -8/3
9.f_x=2x,4x^3
f_y=2y,4y^3
max at sqrt(2), min at 1
11.f_x=3,2x
f_y=2,4y
f_z=4,12z
max at 3.7, min at -3.7
13.f_x=y,2x
f_y=x,2y
f_z=2,2z
max at 20, min at -20
15.f_x=y+z,2x
f_y=x,2y
f_z=x,2z
max at 2sqrt(2), min at -2sqrt(2)