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Algebra Difficulty 6.9 National olympiad Find the answer

Suppose that a,b,ca, b, c, and dd are real numbers simultaneously satisfying
a+bcd=3a + b - c - d = 3
ab3bc+cd3da=4ab - 3bc + cd - 3da = 4
3abbc+3cdda=53ab - bc + 3cd - da = 5
Find 11(ac)2+17(bd)211(a - c)^2 + 17(b -d)^2.

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

Solution

1. We start with the given equations:
a+bcd=3 a + b - c - d = 3
ab3bc+cd3da=4 ab - 3bc + cd - 3da = 4
3abbc+3cdda=5 3ab - bc + 3cd - da = 5

2. From the first equation, we can rewrite it as:
(ac)+(bd)=3 (a - c) + (b - d) = 3
Let x=ac x = a - c and y=bd y = b - d . Thus, we have:
x+y=3 x + y = 3

3. For the second equation, we factor b b out of the first two terms and d d out of the last two terms:
ab3bc+cd3da=b(a3c)+d(c3a)=4 ab - 3bc + cd - 3da = b(a - 3c) + d(c - 3a) = 4

4. For the third equation, we factor b b out of the first two terms and d d out of the last two terms:
3abbc+3cdda=b(3ac)+d(3ca)=5 3ab - bc + 3cd - da = b(3a - c) + d(3c - a) = 5

5. Multiply the third equation by 1-1:
b(c3a)d(a3c)=5 -b(c - 3a) - d(a - 3c) = -5

6. Add the modified third equation to the second equation:
b(a3c)+d(c3a)b(c3a)d(a3c)=45 b(a - 3c) + d(c - 3a) - b(c - 3a) - d(a - 3c) = 4 - 5
Simplifying, we get:
(bd)(a3c)+(bd)(c3a)=1 (b - d)(a - 3c) + (b - d)(c - 3a) = -1
(bd)(a3c+c3a)=1 (b - d)(a - 3c + c - 3a) = -1
(bd)(2a+2c)=1 (b - d)(-2a + 2c) = -1
(bd)(2(ca))=1 (b - d)(2(c - a)) = -1
(bd)(2(x))=1 (b - d)(2(-x)) = -1
2(bd)x=1 -2(b - d)x = -1
2(bd)x=1 2(b - d)x = 1
y2x=1 y \cdot 2x = 1
2xy=1 2xy = 1

7. We now have the system of equations:
x+y=3 x + y = 3
2xy=1 2xy = 1

8. Solving for x x and y y :
y=3x y = 3 - x
Substitute y y into the second equation:
2x(3x)=1 2x(3 - x) = 1
6x2x2=1 6x - 2x^2 = 1
2x26x+1=0 2x^2 - 6x + 1 = 0
Solving this quadratic equation using the quadratic formula x=b±b24ac2a x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} :
x=6±3684 x = \frac{6 \pm \sqrt{36 - 8}}{4}
x=6±284 x = \frac{6 \pm \sqrt{28}}{4}
x=6±274 x = \frac{6 \pm 2\sqrt{7}}{4}
x=3±72 x = \frac{3 \pm \sqrt{7}}{2}

9. Thus, we have two possible pairs (x,y)(x, y):
x=3+72,y=372 x = \frac{3 + \sqrt{7}}{2}, \quad y = \frac{3 - \sqrt{7}}{2}
or
x=372,y=3+72 x = \frac{3 - \sqrt{7}}{2}, \quad y = \frac{3 + \sqrt{7}}{2}

10. We need to find 11(ac)2+17(bd)2 11(a - c)^2 + 17(b - d)^2 :
11x2+17y2 11x^2 + 17y^2
Substituting x x and y y :
11(3+72)2+17(372)2 11 \left( \frac{3 + \sqrt{7}}{2} \right)^2 + 17 \left( \frac{3 - \sqrt{7}}{2} \right)^2
11(9+67+74)+17(967+74) 11 \left( \frac{9 + 6\sqrt{7} + 7}{4} \right) + 17 \left( \frac{9 - 6\sqrt{7} + 7}{4} \right)
11(16+674)+17(16674) 11 \left( \frac{16 + 6\sqrt{7}}{4} \right) + 17 \left( \frac{16 - 6\sqrt{7}}{4} \right)
11(4+372)+17(4372) 11 \left( 4 + \frac{3\sqrt{7}}{2} \right) + 17 \left( 4 - \frac{3\sqrt{7}}{2} \right)
44+3372+685172 44 + \frac{33\sqrt{7}}{2} + 68 - \frac{51\sqrt{7}}{2}
11297 112 - 9\sqrt{7}

The final answer is 63\boxed{63}.

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