Let f(x)=xD(x)

Starting from the definition of derivative

X tends to 0, and f'(0)=limD(x), so f(x)=xD(x) is not derivable at x=0.

Let f(x)=x? D(x), from the boundedness of D(x), we can know that f(x)=x? D(x) is derivable only when x=0.

When x0=0, f' (0) = limxd (x) = 0, and f (x) = x, x tends to 0? D(x) is differentiable at x=0.

When x0≠0, f(x) is discontinuous at x=x0, so f(x) is not derivable at x=x0.

(1) is only differentiable at one point and discontinuous at other points. f(x)=(x-a)？ D(x)

(2) It is only derivable at a limited number of points, but discontinuous at other points. f(x)=(x-a 1)？ (x-a2)？ ……(x-an)？ D(x)