Assignment 2....headache

I just started assignment 2 and I feel that it's alot harder than the first one.

The first question, I feel that we have met the hardest question about a tree.
I have a little bit insight about small part of the formula,
but I still feel headache about drawing trees.

The second was easy to unwind the expression. However the closed form is
not easy. I'm not sure whether we should let it smaller than certain number
or let it equal to an expression. Also, the constant ting annoys me.

I didn't start reviewing the next 2 quesion but I hope they will be easier.