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Algebra Difficulty 6.0 AIME, harder Prove it South Africa

Find all pairs of real numbers xx and yy which satisfy the following equations:
x2+y248x29y+714=02xy29x48y+756=0 \begin{aligned} x^2 + y^2 - 48x - 29y + 714 &= 0 \\ 2xy - 29x - 48y + 756 &= 0 \end{aligned}

Solutions — 2

Solution 1

The two equations can be rewritten as
(x24)2+(y292)2=2894(x24)(y292)=30. \begin{aligned} (x - 24)^2 + \left(y - \frac{29}{2}\right)^2 &= \frac{289}{4} \\ (x - 24)\left(y - \frac{29}{2}\right) &= -30. \end{aligned}
By putting X=x24X = x - 24 and Y=y292Y = y - \frac{29}{2}, we obtain
X2+Y2=2894(1) X^2 + Y^2 = \frac{289}{4} \tag{1}
XY=30.(2) XY = -30. \tag{2}
From (2) we have Y=30/XY = -30/X. Plug this into (1) to get X2+900X2=2894X^2 + \frac{900}{X^2} = \frac{289}{4}, which simplifies to
X42894X2+900=0. X^4 - \frac{289}{4}X^2 + 900 = 0.
We can now solve for X2X^2, using the quadratic formula:
X2=12(2894±(2894)2602)=12(2894±(289460)(2894+60))=12(2894±4945294)=12(289±1614), \begin{aligned} X^2 &= \frac{1}{2} \left( \frac{289}{4} \pm \sqrt{\left(\frac{289}{4}\right)^2 - 60^2} \right) \\ &= \frac{1}{2} \left( \frac{289}{4} \pm \sqrt{\left(\frac{289}{4} - 60\right) \left(\frac{289}{4} + 60\right)} \right) \\ &= \frac{1}{2} \left( \frac{289}{4} \pm \sqrt{\frac{49}{4} \cdot \frac{529}{4}} \right) \\ &= \frac{1}{2} \left( \frac{289 \pm 161}{4} \right), \end{aligned}
so that X2=2254X^2 = \frac{225}{4} or X2=16X^2 = 16. Thus, X=±152X = \pm\frac{15}{2} or X=±4X = \pm 4, and correspondingly, Y=4Y = \mp 4 or Y=152Y = \mp\frac{15}{2}. Using x=X+24x = X + 24 and y=Y+292y = Y + \frac{29}{2}, we find that (x,y)(x, y) solves the original set of equations if and only if (x,y){(28,7),(20,22),(332,372),(632,212)}(x, y) \in \{(28, 7), (20, 22), (\frac{33}{2}, \frac{37}{2}), (\frac{63}{2}, \frac{21}{2})\}.

Solution 2

Proceed as in Solution 1 up to the two equations
X2+Y2=2894(1) X^2 + Y^2 = \frac{289}{4} \tag{1}
XY=30.(2) XY = -30. \tag{2}
By either adding 2XY=602XY = -60 to (1), or subtracting it from (1), we get, respectively,
X2+2XY+Y2=(X+Y)2=494=(72)2X22XY+Y2=(XY)2=5294=(232)2. \begin{aligned} X^2 + 2XY + Y^2 &= (X + Y)^2 = \frac{49}{4} = \left(\frac{7}{2}\right)^2 \\ X^2 - 2XY + Y^2 &= (X - Y)^2 = \frac{529}{4} = \left(\frac{23}{2}\right)^2. \end{aligned}
From this, we have to solve four systems of equations:
X+Y=±72XY=±232 \begin{aligned} X + Y &= \pm\frac{7}{2} \\ X - Y &= \pm\frac{23}{2} \end{aligned}

X + YX - YXYxy
72\frac{7}{2}232\frac{23}{2}152\frac{15}{2}4-4632\frac{63}{2}212\frac{21}{2}
72\frac{7}{2}232-\frac{23}{2}4-4152\frac{15}{2}20202222
72-\frac{7}{2}232\frac{23}{2}44152-\frac{15}{2}282877
72-\frac{7}{2}232-\frac{23}{2}152-\frac{15}{2}44332\frac{33}{2}372\frac{37}{2}

We conclude that there are exactly four pairs (x,y)(x, y) of real numbers that solve the original two equations, namely (632,212)(\frac{63}{2}, \frac{21}{2}), (20,22)(20, 22), (28,7)(28, 7) and (332,372)(\frac{33}{2}, \frac{37}{2}).

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