6) Let ƒ be defined by f(x)=\left\{\begin{array}{ll} x^{4}\left(2+\sin \frac{1}{x}\right) & x \neq 0 \\ 0 & x=0 \end{array}\right. (a) Show that ƒ has absolute minimum at x = 0.%3D (b)

Show that f'(x)assumes both negative and positive values in every neighborhood of