Hello again!

I want to bring up my issue one more time to finally solve it! I want to understand how it works.

Here is what I have:

I want to calculate the left derivate.

I got from the integral which is (1)

The second inequality I get is from the advice from girdav which was to consider

This inequality can be put to and it leads to (2)

I got (1) and (2). How can I use these two inequalities for the left derivate? If I can replace the in (1) by something else which is bigger than k I would get the same derivate.

My conclusions should be correct until here but I donīt see how I can proceed from there.

I need a hint and I would be very thankful for that little hint to finally finish the work.

Thanks.