1998 AIME Problem 14

Below is the professionally curated solution for Problem 14 of the 1998 AIME, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 1998 AIME solutions, or check the answer key.

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Concepts:Diophantine EquationSimon’s Favorite Factoring Trickbounding to limit cases

Difficulty rating: 2740

14.

An m×n×pm \times n \times p rectangular box has half the volume of an (m+2)×(n+2)×(p+2)(m + 2) \times (n + 2) \times (p + 2) rectangular box, where m,m, n,n, and pp are integers, and mnp.m \le n \le p. What is the largest possible value of p?p?

Solution:

The condition 2mnp=(m+2)(n+2)(p+2)2mnp = (m + 2)(n + 2)(p + 2) rewrites as (1+2m)(1+2n)(1+2p)=2.\left(1 + \frac{2}{m}\right)\left(1 + \frac{2}{n}\right)\left(1 + \frac{2}{p}\right) = 2. If m=1m = 1 the first factor alone is 3>2,3 \gt 2, and if m=2m = 2 it equals 22 while the other factors exceed 1;1; both are impossible. If m5,m \ge 5, then since nmn \ge m the first two factors are at most (75)2=4925,\left(\frac{7}{5}\right)^2 = \frac{49}{25}, forcing 1+2p5049,1 + \frac{2}{p} \ge \frac{50}{49}, i.e. p98.p \le 98.

For m=4m = 4 the equation becomes 4np=3(n+2)(p+2),4np = 3(n + 2)(p + 2), i.e. (n6)(p6)=48,(n - 6)(p - 6) = 48, so p54.p \le 54. For m=3m = 3 it becomes 6np=5(n+2)(p+2),6np = 5(n + 2)(p + 2), i.e. np10n10p20=0,np - 10n - 10p - 20 = 0, or (n10)(p10)=120.(n - 10)(p - 10) = 120. Both factors must be positive (if n,p<10n, p \lt 10 the product (10n)(10p)(10 - n)(10 - p) is at most 4949), so the largest pp comes from n10=1:n - 10 = 1: n=11n = 11 and p=130.p = 130. Indeed 2311130=8580=513132.2 \cdot 3 \cdot 11 \cdot 130 = 8580 = 5 \cdot 13 \cdot 132.

Since every other case yields p98,p \le 98, the largest possible value is p=130.p = 130.

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