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Ukrainian National Mathematical Olympiad

Algebra Difficulty 4.6 AIME Find the answer Ukraine

Are there integers a<b<c<da < b < c < d such that
aa+ab+ac+ad=ba+bb+bc+bd? \frac{a}{a} + \frac{a}{b} + \frac{a}{c} + \frac{a}{d} = \frac{b}{a} + \frac{b}{b} + \frac{b}{c} + \frac{b}{d} ?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Answer. Yes, for example a=−28a = -28, b=−14b = -14, c=−7c = -7 and d=4d = 4.

Clearly, all numbers cannot be positive. Let's take a=−4a = -4, b=−2b = -2, c=−1c = -1 and find the corresponding dd:
−4−4+−4−2+−4−1+−4d=−2−4+−2−2+−2−1+−2d1+2+4+−4d=12+1+2+−2d7−4d=72−2d7−4d+2d=727−2d=727−72=2d72=2dd=47. \begin{aligned} \frac{-4}{-4} + \frac{-4}{-2} + \frac{-4}{-1} + \frac{-4}{d} &= \frac{-2}{-4} + \frac{-2}{-2} + \frac{-2}{-1} + \frac{-2}{d} \\ 1 + 2 + 4 + \frac{-4}{d} &= \frac{1}{2} + 1 + 2 + \frac{-2}{d} \\ 7 - \frac{4}{d} &= \frac{7}{2} - \frac{2}{d} \\ 7 - \frac{4}{d} + \frac{2}{d} &= \frac{7}{2} \\ 7 - \frac{2}{d} = \frac{7}{2} \\ 7 - \frac{7}{2} = \frac{2}{d} \\ \frac{7}{2} = \frac{2}{d} \\ d = \frac{4}{7}. \end{aligned}

The equality is homogeneous, meaning that we can simply multiply all numbers by 77 to obtain the aforementioned integers.

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