Python story--three (attached)

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Many of the problems caused by the Fibonacci sequence know that a lot of knowledge needs to be considered and summed up
For the math is OK, the algorithm has the basis of a look to understand, but some people do not understand, the final step by step to do the analysis is clear, the principle of understanding the grammar you are free:
Based on recursion (Fibonacci that tangent sequence):

In fact, this is so simple, handwriting a bit messy, with the number in it, after all, when recursive to the last n<=2 will have a return value (also conform to recursive 1, call the function itself 2, there is a return value), inappropriate description is to peel onions, but also a layer of release.
So much so that there are many kinds of lambda,while in grammar.

Python story--three (attached)