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. (Enter your answers as a comma-separated list. If an answer does not exist, enter DNE.)

a) f(x, y) = 3 2x + 4y x² 4y²

^b)f(x, y) = 6 x⁴ + 2x² y²

^c)f(x, y) = 3x³ 9x + 9xy²

^d)f(x, y) = x³ + y³ 3x² 9y² 9x

e) f(x, y) = xy + 27/x + 27/y

f) f(x, y) = y² 4y cos(x), 1 x 7

g) f(x, y) = 2 sin(x) sin(y), < x < , < y <