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Algebra Difficulty 6.0 AIME, harder Prove it Ukraine

Distinct real numbers aa, bb, cc satisfy the condition a+1b=b+1c=c+1aa + \frac{1}{b} = b + \frac{1}{c} = c + \frac{1}{a}. Find which values the product abcabc can attain.

Solution

From the first equality we can get that ab=1c1ba-b = \frac{1}{c} - \frac{1}{b} or that (ab)=bcbc(a-b) = \frac{b-c}{bc}.
Analogously bc=caacb-c = \frac{c-a}{ac} and ca=ababc-a = \frac{a-b}{ab}. Thus we obtain: ab=ab(abc)2a-b = \frac{a-b}{(abc)^2}. Since aa, bb are distinct, (abc)2=1(abc)^2 = 1, which means that abc=±1abc = \pm 1. Let us show that there exist numbers aa, bb, cc for which both of the values can be achieved.

Take a=1a = 1. Then we have two equations on bb, cc: 1+1b=c+11 + \frac{1}{b} = c + 1 and 1+1b=b+1c1 + \frac{1}{b} = b + \frac{1}{c}.
From the first one we get that c=1b2b=1+1bc = \frac{1}{b} \Rightarrow 2b = 1 + \frac{1}{b} or 2b2b1=02b^2 - b - 1 = 0. This equation has two roots b1=12-b_1 = -\frac{1}{2} and b2=1b_2 = 1. The second root coincides with aa, thus we take b=12b = -\frac{1}{2} and c=2c = -2. For these aa, bb, cc we have abc=1abc = 1 and it is easy to check that they satisfy the problem conditions.

Now take a=11+1b=c1a = -1 \Rightarrow -1 + \frac{1}{b} = c - 1 and 1+1b=b+1cc=1b-1 + \frac{1}{b} = b + \frac{1}{c} \Rightarrow c = \frac{1}{b} and 2b=1b12b = \frac{1}{b} - 1. So 2b2+b1=02b^2 + b - 1 = 0. The roots of this equation are b1=12b_1 = \frac{1}{2} and b2=1b_2 = -1. Again we choose only one value b=12c=2b = \frac{1}{2} \Rightarrow c = 2 and abc=1abc = -1. A direct check shows that these aa, bb, cc fulfill the conditions.

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