Question

a) Find a recurrence relation for the number of ways to climb n stairs if the person climbing the stairs can take one, two, or three stairs at a time. b) What are the initial conditions? c) In many ways can this person climb a flight of eight stairs?

Solution

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(a) Let ana_n represent the number of ways to climb nn stairs if the person climbing the stairs can take one, two, or three stairs at a time.

First case\textbf{First case} The final step is taken by 1 stair, then there are an1a_{n-1} ways to climb the first n1n-1 steps.

Second case\textbf{Second case} The final step is taken by 2 stairs, then there are an2a_{n-2} ways to climb the first n2n-2 steps.

Third case\textbf{Third case} The final step is taken by 3 stairs, then there are an3a_{n-3} ways to climb the first n3n-3 steps.

Adding the number of sequences of all three cases, we then obtain:

an=an1+an2+an3a_n=a_{n-1}+a_{n-2}+a_{n-3}

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