There is nothing to understand, it is a formula. If can be derived recursively from the concept above it. But both the derivation and the formula itself are time-consuming and impractical, so it is better to not bother with that 🙂 Hence the other methods mentioned below!
CAT-holics 
how simply u explain …the topic which was very difficult for me …thanks a lot 🙂 🙂
i did not understand the 2nd case above
There is nothing to understand, it is a formula. If can be derived recursively from the concept above it. But both the derivation and the formula itself are time-consuming and impractical, so it is better to not bother with that 🙂 Hence the other methods mentioned below!
regards
J