Table Legs

Algebra Level pending

Legs L 1 L_1 , L 2 L_2 , L 3 L_3 , L 4 L_4 of a square table each have length n n , where n n is a positive integer. For how many ordered 4-tuples ( k 1 , k 2 , k 3 , k 4 ) (k_1, k_2, k_3, k_4) of nonnegative integers can we cut a piece of length k i k_i from the end of leg L i L_i ( i = 1 , 2 , 3 , 4 ) (i=1,2,3,4) and still have a stable table?

(The table is stable if it can be placed so that all four of the leg ends touch the floor. Note that a cut leg of length 0 is permitted.)

Source: 2005 USAMO Problem 4

2 n 3 + 6 n 2 + 3 n + 7 6 \frac{2n^3+6n^2+3n+7}{6} n 3 + 6 n 2 + 7 n + 4 6 \frac{n^3+6n^2+7n+4}{6} n 3 + 6 n 2 + 5 n + 6 6 \frac{n^3+6n^2+5n+6}{6} 2 n 3 + 6 n 2 + 7 n + 3 3 \frac{2n^3+6n^2+7n+3}{3} 2 n 3 + 6 n 2 + 7 n + 3 6 \frac{2n^3+6n^2+7n+3}{6}

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