f(x) = (x - 3)^3
d/dx( f(x) ) = 3(x-3)^2
d^2/dx^2 ( f(x) ) = 6(x -3)
d/dx of f(x) = 0 when x =3
but d^2/dx^2 is also zero
thus we need to take a point to the left and right of 3 and left of 3
d/dx(f(x)) for x = 2.5 and 3.5 are both positive since 3(2.5 -3)^2 and 3(3.5 -3)^2 are positive
thus the function has no minima or maxima, x =3 is a point of inflection
