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Geometry Difficulty 5.8 AIME, harder Prove it Belarus

Three points AA, BB, CC, are marked on the hyperbola y=1/xy = 1/x so that the triangle ABCABC is equilateral.
Find all possible values of the product of the sum of abscissae and the sum of ordinates of the vertices of ABCABC.

Solution

Answer 9.
Let A(a;1/a)A(a; 1/a), B(b;1/b)B(b; 1/b), C(c;1/c)C(c; 1/c) be the marked points. Then the required product is equal to
T=(a+b+c)(1a+1b+1c). T = (a + b + c) \left( \frac{1}{a} + \frac{1}{b} + \frac{1}{c} \right).
Figure 1
Since any vertical and any horizontal line meets the hyperbola y=1/xy = 1/x at most at one point we see that the numbers a,b,ca, b, c are pairwise distinct. By condition, the triangle ABCABC is regular so
AB=BC=CA    (ba)2+(1/b1/a)2==(cb)2+(1/c1/b)2=(ac)2+(1/a1/c)2.(1) \begin{aligned} AB = BC = CA &\iff (b-a)^2 + (1/b - 1/a)^2 = \\ &= (c-b)^2 + (1/c - 1/b)^2 = (a-c)^2 + (1/a - 1/c)^2. \end{aligned} \quad (1)
From (1) we obtain
b22ab+a2+1/b22/(ab)+1/a2=c22bc+b2+1/c22/(bc)+1/b2    (a2c2)2b(ac)+c2a2a2c22(ca)abc=0. \begin{aligned} b^2 - 2ab + a^2 + 1/b^2 - 2/(ab) + 1/a^2 &= c^2 - 2bc + b^2 + 1/c^2 - 2/(bc) + 1/b^2 \\ &\implies (a^2 - c^2) - 2b(a-c) + \frac{c^2 - a^2}{a^2c^2} - \frac{2(c-a)}{abc} = 0. \end{aligned}
Since aca \neq c we have
a+c2ba+ca2c2+2abc=0. a+c-2b-\frac{a+c}{a^2c^2}+\frac{2}{abc}=0.
In a similar way from (1) we obtain two more equalities
b+a2cb+ab2a2+2abc=0andc+b2ac+bc2b2+2abc=0. b+a-2c-\frac{b+a}{b^2a^2}+\frac{2}{abc}=0 \quad \text{and} \quad c+b-2a-\frac{c+b}{c^2b^2}+\frac{2}{abc}=0.
Summing all three equalities, we obtain 6abca+ca2c2b+ab2a2c+bc2b2=0\frac{6}{abc} - \frac{a+c}{a^2c^2} - \frac{b+a}{b^2a^2} - \frac{c+b}{c^2b^2} = 0, so bc+ba+ca+cb+ab+ac=6\frac{b}{c} + \frac{b}{a} + \frac{c}{a} + \frac{c}{b} + \frac{a}{b} + \frac{a}{c} = 6. Thus
T=(a+b+c)(1a+1b+1c)=1+ab+ac+ba+1+bc+ca+cb+1=3+6=9. T = (a+b+c) \left( \frac{1}{a} + \frac{1}{b} + \frac{1}{c} \right) = 1 + \frac{a}{b} + \frac{a}{c} + \frac{b}{a} + 1 + \frac{b}{c} + \frac{c}{a} + \frac{c}{b} + 1 = 3 + 6 = 9.

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