Check if the following functions have any maxima or minima:

i. f(a,b,c) = a² + 3b² - 3ab + 4bc + 6c²

ii. f(a,b) = b² - a²

iii. f(a,b) = a⁵b + ab⁵ + ab

iv. f(a,b) = log(a² + b² + 1)