Maths Olympiad Prep

Library / /353 of 520

Algebra Difficulty 5.8 AIME, harder Find the answer

4-158 Try to find the integer solutions of the following system of equations
{xx+y=y60yx+y=x15 \left\{\begin{array}{l} x^{x+y}=y^{60} \\ y^{x+y}=x^{15} \end{array}\right.

A number or a short expression. Spacing and $ signs are ignored.

Solution

[Solution] Since y(x+y)2=x15(x+y)=(y60)15=y900y^{(x+y)^{2}}=x^{15(x+y)}=\left(y^{60}\right)^{15}=y^{900}. Therefore, y=±1y= \pm 1 or (x+y)2=900(x+y)^{2}=900, i.e., x+y=±30x+y= \pm 30. Substituting y=±1y= \pm 1 into the second equation of the original system, we have
(±1)x±1=x15,x=1, ( \pm 1)^{x \pm 1}=x^{15},|x|=1,

but x=1x=-1 is not a solution, while x=1,y=±1x=1, y= \pm 1 are two sets of integer solutions.
When x+y=30x+y=-30, from the first equation of the original system, we get x=y2x=y^{-2}, substituting into x+y=x+y= -30, we have y3+30y2+1=0y^{3}+30 y^{2}+1=0 which has no integer solutions.
When x+y=30x+y=30, substituting x=y2x=y^{2}, we get y2+y30=0y^{2}+y-30=0, which gives y=5y=5 and y=6y=-6, with corresponding x=25x=25 and x=36x=36, providing two more sets of integer solutions. Therefore, the original equation has four sets of integer solutions:
{x=1,y=1;{x=1,y=1;{x=25,y=5;{x=36y=6. \left\{\begin{array} { l } { x = 1 , } \\ { y = 1 ; } \end{array} \quad \left\{\begin{array} { l } { x = 1 , } \\ { y = - 1 ; } \end{array} \quad \left\{\begin{array} { l } { x = 2 5 , } \\ { y = 5 ; } \end{array} \quad \left\{\begin{array}{l} x=36 \\ y=-6 . \end{array}\right.\right.\right.\right.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

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