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Pascal row with neighbouring entries in ratio 3 to 4 to 5

Question

Some row nn of Pascal's triangle contains three adjacent entries whose ratio is 3:4:53:4:5. What is the smallest such nn?

Short answer

6262. Write the entries as (nk−1),(nk),(nk+1)\binom{n}{k-1},\binom nk,\binom n{k+1}. Their consecutive ratios are kn−k+1=34\tfrac{k}{n-k+1}=\tfrac34 and k+1n−k=45\tfrac{k+1}{n-k}=\tfrac45, i.e. 7k=3n+37k=3n+3 and 9k=4n−59k=4n-5, whose unique solution is n=62n=62, k=27k=27.

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