Maths Olympiad Prep

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Algebra Difficulty 5.0 AIME Find the answer

For how many triples (x,y,z)(x, y, z) of integers between -10 and 10 inclusive do there exist reals a,b,ca, b, c that satisfy ab=xac=ybc=z?\begin{gathered} a b=x \\ a c=y \\ b c=z ? \end{gathered}

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

Solution

If none are of x,y,zx, y, z are zero, then there are 4103=40004 \cdot 10^{3}=4000 ways, since xyzx y z must be positive. Indeed, (abc)2=xyz(a b c)^{2}=x y z. So an even number of them are negative, and the ways to choose an even number of 3 variables to be negative is 4 ways. If one of x,y,zx, y, z is 0 , then one of a,b,ca, b, c is zero at least. So at least two of x,y,zx, y, z must be 0 . If all 3 are zero, this gives 1 more solution. If exactly 2 are negative, then this gives 3203 \cdot 20 more solutions. This comes from choosing one of x,y,zx, y, z to be nonzero, and choosing its value in 20 ways. Our final answer is 4000+60+1=40614000+60+1=4061.

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