Section 3.2 #9
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Hi Kamil. This mostly looks very good. I do have two comments, though.
First, when you’re showing that
is 
, you have 3 inequalities that hold when 
; the right-hand-side of each is 
. When you add up the left sides of these inequalities, you get 
but when you add up the right side, you get 
which is 
. This tells you that for 
, you have 
. But you have written 
. Following this through, your value of 
should be 
, not 
.
Next, when you’re showing that
is not 
, it looks like you’ve shown that the particular choice 
does not work. This is not enough to show that 
is not 
. Instead you want to show that there are NO POSSIBLE CHOICES of 
that work. Try setting up a proof by contradiction to see this formally. (BTW I added a comment to Christian’s post that included a proof by contradiction for another set of functions…that might be useful for you to look at.)