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 ðŸ™‚ Hence the other methods mentioned below!

how simply u explain …the topic which was very difficult for me …thanks a lot ðŸ™‚ ðŸ™‚

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 ðŸ™‚ Hence the other methods mentioned below!

regards

J