Maths Olympiad Prep

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Algebra Difficulty 6.2 National Olympiad Prove it Philippines

Problem:
Find all real solutions to the system of equations
{x(y1)+y(x+1)=6(x1)(y+1)=1 \begin{cases} x(y-1) + y(x+1) = 6 \\ (x-1)(y+1) = 1 \end{cases}

Solution

Solution:
Let us first expand and simplify the equations.

From the first equation:
x(y1)+y(x+1)=6 x(y-1) + y(x+1) = 6
Expand:
xyx+yx+y=6 xy - x + yx + y = 6
Combine like terms:
xyx+xy+y=6 xy - x + xy + y = 6
2xyx+y=6 2xy - x + y = 6

From the second equation:
(x1)(y+1)=1 (x-1)(y+1) = 1
Expand:
xy+xy1=1 xy + x - y - 1 = 1
xy+xy=2 xy + x - y = 2

Now, let us solve the system:

We have:
1) 2xyx+y=62xy - x + y = 6
2) xy+xy=2xy + x - y = 2

Let us add the two equations:

Equation 1: 2xyx+y=62xy - x + y = 6
Equation 2: xy+xy=2xy + x - y = 2

Let us try to eliminate variables. Add equation 1 and equation 2:

But first, let's try to express one variable in terms of the other.

From equation 2:
xy+xy=2 xy + x - y = 2
xyy=2x xy - y = 2 - x
y(x1)=2x y(x - 1) = 2 - x
If x1x \neq 1,
y=2xx1 y = \frac{2 - x}{x - 1}

Now substitute this value of yy into equation 1:
2xyx+y=6 2xy - x + y = 6
Substitute yy:
2x(2xx1)x+2xx1=6 2x \left(\frac{2 - x}{x - 1}\right) - x + \frac{2 - x}{x - 1} = 6
Multiply both sides by x1x - 1 (assuming x1x \neq 1):
2x(2x)x(x1)+(2x)=6(x1) 2x(2 - x) - x(x - 1) + (2 - x) = 6(x - 1)
Expand:
2x(2x)=4x2x2 2x(2 - x) = 4x - 2x^2
x(x1)=x2+x-x(x - 1) = -x^2 + x
So:
4x2x2x2+x+2x=6x6 4x - 2x^2 - x^2 + x + 2 - x = 6x - 6
Combine like terms:
2x2x2=3x2-2x^2 - x^2 = -3x^2
4x+xx=4x4x + x - x = 4x
So:
3x2+4x+2=6x6 -3x^2 + 4x + 2 = 6x - 6
Bring all terms to one side:
3x2+4x+26x+6=0 -3x^2 + 4x + 2 - 6x + 6 = 0
3x22x+8=0 -3x^2 - 2x + 8 = 0
Multiply both sides by 1-1:
3x2+2x8=0 3x^2 + 2x - 8 = 0
Now solve this quadratic equation:
x=2±4+966=2±1006=2±106 x = \frac{-2 \pm \sqrt{4 + 96}}{6} = \frac{-2 \pm \sqrt{100}}{6} = \frac{-2 \pm 10}{6}
So:
x1=2+106=86=43 x_1 = \frac{-2 + 10}{6} = \frac{8}{6} = \frac{4}{3}
x2=2106=126=2 x_2 = \frac{-2 - 10}{6} = \frac{-12}{6} = -2
Now, recall y=2xx1y = \frac{2 - x}{x - 1}.

For x=43x = \frac{4}{3}:
y=243431=643433=2313=2 y = \frac{2 - \frac{4}{3}}{\frac{4}{3} - 1} = \frac{\frac{6 - 4}{3}}{\frac{4 - 3}{3}} = \frac{\frac{2}{3}}{\frac{1}{3}} = 2
For x=2x = -2:
y=2(2)21=43=43 y = \frac{2 - (-2)}{-2 - 1} = \frac{4}{-3} = -\frac{4}{3}

Therefore, the real solutions are:
(x,y)=(43,2),(x,y)=(2,43) (x, y) = \left(\frac{4}{3}, 2\right),\quad (x, y) = \left(-2, -\frac{4}{3}\right)

Check for x=1x = 1:
From above, y=2xx1y = \frac{2 - x}{x - 1} is undefined for x=1x = 1, so no solution for x=1x = 1.

Final Answer:
All real solutions are:
(43,2), (2,43) \boxed{\left(\frac{4}{3}, 2\right),\ \left(-2, -\frac{4}{3}\right)}

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