Determine whether the following two functions are differentiable at the given value

a) f(x) = xsin(1/x), f(0)=0: at given value 0

b)

f(x) = 1-x if x<1

x(1-x) if x>= 1 at given value 1

I think a is differentiable. I found the difference quotient and found that as

h->0 the limit of the quotient -> 0 also.

But I can't seem to get started on b!