Olympiad Maths Prep

Track / Stage 5 / 382 of 400 #982 of 2000

Problem 982

AIME late
Combinatorics Difficulty 6.0 Find the answer

Example 4 Find the number of positive integer solutions to the indeterminate equation
x+2y+3z=2012x+2 y+3 z=2012

Official solution

Let (x,y,z)(x, y, z) be a positive integer solution to (1), then 3z20093 z \leqslant 2009, i.e., 1z6691 \leqslant z \leqslant 669, which respectively yield
x+2y=2009,2006,,5x+2 y=2009,2006, \cdots, 5

Correspondingly, the range of values for yy are
1y1004,1y1002,1y10011y999,,1y2\begin{array}{l} 1 \leqslant y \leqslant 1004,1 \leqslant y \leqslant 1002,1 \leqslant y \leqslant 1001 \\ 1 \leqslant y \leqslant 999, \cdots, 1 \leqslant y \leqslant 2 \end{array}

Since when yy and zz are determined, the value of xx is uniquely determined, the number of positive integer solutions to (1) is
(1004+1002)+(1001+999)++(5+3)+2=2006+2000++8+2=12(2006+2)×335=336340\begin{aligned} & (1004+1002)+(1001+999)+\cdots+(5+3)+2 \\ = & 2006+2000+\cdots+8+2 \\ = & \frac{1}{2}(2006+2) \times 335=336340 \end{aligned}

In summary, there are 336340 sets of positive integer solutions.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.