Monopoly firm's block pricing strategy making max. profit I have question about max. profit condition for monopolist block pricing.  

I learned that to maximize profit in block pricing, given a linear demand curve function and constant MC, calculate $Q_2(Q_1)$, and $P_S(Q_1)$ then find $Q_1$ maximizing profit and then find $P_1$ and $Q_2$ the $P_2$.

This is what I read from the textbook (Microeconomic, Besanko).  

But I heard somewhere that when in following case,



with right-downward sloping $MC$ and $AC$,

Profit maximizing first block price and quantity is $P_1$ and $Q_1$ where $AC=MR$
and second block price and quantity should be $P_2$ and $Q_2$ where $AC=D$

I've thought about this for 1 day and still couldn't get why maximizing condition is like that.
The reason why the profit maximising condition is like that is because the monopoly is Price Discriminating between those who are willing to pay at  $P_1$ and $P_2$ for $Q_1$ and $Q_2$ respectively.

This increases the area of producer surplus to an area greater than what it would be by charging $P_1$ and $P_2$ alone.