PPT Slide

An important identity for binomial coefficients

Combinatorial proof? Consider separating all k-element subsets of the set {1,2,…,n} into those that include and those that exclude n

Graphical intuition:

Let n=x+y be the total

steps needed in the city walk problem, and let k=y be the

number of northward steps.

There are two cases for the

very last step taken.

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