AlgebraDifficulty 6.9National olympiadFind the answer
Suppose that a,b,c, and d are real numbers simultaneously satisfying a+b−c−d=3 ab−3bc+cd−3da=4 3ab−bc+3cd−da=5 Find 11(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+b−c−d=3 ab−3bc+cd−3da=4 3ab−bc+3cd−da=5
2. From the first equation, we can rewrite it as: (a−c)+(b−d)=3 Let x=a−c and y=b−d. Thus, we have: x+y=3
3. For the second equation, we factor b out of the first two terms and d out of the last two terms: ab−3bc+cd−3da=b(a−3c)+d(c−3a)=4
4. For the third equation, we factor b out of the first two terms and d out of the last two terms: 3ab−bc+3cd−da=b(3a−c)+d(3c−a)=5
5. Multiply the third equation by −1: −b(c−3a)−d(a−3c)=−5
6. Add the modified third equation to the second equation: b(a−3c)+d(c−3a)−b(c−3a)−d(a−3c)=4−5 Simplifying, we get: (b−d)(a−3c)+(b−d)(c−3a)=−1 (b−d)(a−3c+c−3a)=−1 (b−d)(−2a+2c)=−1 (b−d)(2(c−a))=−1 (b−d)(2(−x))=−1 −2(b−d)x=−1 2(b−d)x=1 y⋅2x=1 2xy=1
7. We now have the system of equations: x+y=3 2xy=1
8. Solving for x and y: y=3−x Substitute y into the second equation: 2x(3−x)=1 6x−2x2=1 2x2−6x+1=0 Solving this quadratic equation using the quadratic formula x=2a−b±b2−4ac: x=46±36−8 x=46±28 x=46±27 x=23±7
9. Thus, we have two possible pairs (x,y): x=23+7,y=23−7 or x=23−7,y=23+7
10. We need to find 11(a−c)2+17(b−d)2: 11x2+17y2 Substituting x and y: 11(23+7)2+17(23−7)2 11(49+67+7)+17(49−67+7) 11(416+67)+17(416−67) 11(4+237)+17(4−237) 44+2337+68−2517 112−97
The final answer is 63.
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