Question

Can someone explain to me why the critical # is 1 and not 0?

Answer 1

http://c782279.r79.cf2.rackcdn.com/se03c01041.png

Answer 2

Because f'(x) = -2x only for x>1 so you can't substitute 0 into that to get a critical point.
Critical points are places where f'(x)=0 or where f'(x) does not exist. If you take a look at x=1, the derivative from the left is 3. From the right it's -2. So f'(x) does not exist at x=1. Therefore it's a critical point

Answer 3

@Xavier Awesome, thanks!