AlgebraDifficulty 6.0AIME, harderProve itSouth Africa
Find all pairs of real numbers x and y which satisfy the following equations: x2+y2−48x−29y+7142xy−29x−48y+756=0=0
Solutions — 2
Solution 1
The two equations can be rewritten as (x−24)2+(y−229)2(x−24)(y−229)=4289=−30. By putting X=x−24 and Y=y−229, we obtain X2+Y2=4289(1) XY=−30.(2) From (2) we have Y=−30/X. Plug this into (1) to get X2+X2900=4289, which simplifies to X4−4289X2+900=0. We can now solve for X2, using the quadratic formula: X2=214289±(4289)2−602=21(4289±(4289−60)(4289+60))=21(4289±449⋅4529)=21(4289±161), so that X2=4225 or X2=16. Thus, X=±215 or X=±4, and correspondingly, Y=∓4 or Y=∓215. Using x=X+24 and y=Y+229, we find that (x,y) solves the original set of equations if and only if (x,y)∈{(28,7),(20,22),(233,237),(263,221)}.
Solution 2
Proceed as in Solution 1 up to the two equations X2+Y2=4289(1) XY=−30.(2) By either adding 2XY=−60 to (1), or subtracting it from (1), we get, respectively, X2+2XY+Y2X2−2XY+Y2=(X+Y)2=449=(27)2=(X−Y)2=4529=(223)2. From this, we have to solve four systems of equations: X+YX−Y=±27=±223
X + Y
X - Y
X
Y
x
y
27
223
215
−4
263
221
27
−223
−4
215
20
22
−27
223
4
−215
28
7
−27
−223
−215
4
233
237
We conclude that there are exactly four pairs (x,y) of real numbers that solve the original two equations, namely (263,221), (20,22), (28,7) and (233,237).
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