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2000 AMC 12 Problems/Problem 20

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Problem

If x,y, and z are positive numbers satisfying

x + 1/y = 4,\qquad y + 1/z = 1, \qquad \text{and} \qquad z + 1/x = 7/3

Then what is the value of xyz ?

\text {(A)}\ 2/3 \qquad \text {(B)}\ 1 \qquad \text {(C)}\ 4/3 \qquad \text {(D)}\ 2 \qquad \text {(E)}\ 7/3

Contents


Solution

Solution 1

Multiplying all three expressions together,

\begin{align*}\left( x + \frac 1y \right) \left( y + \frac 1z \right) \left( z + \frac 1x \right) &= xyz + x + y + z + \f...

Thus xyz = 1 \Rightarrow B

Solution 2

We have a system of three equations and three variables, so we can apply repeated substitution.

4 = x + \frac{1}{y} = x + \frac{1}{1 - \frac{1}{z}} = x + \frac{1}{1-\frac{1}{7/3-1/x}} = x + \frac{7x-3}{4x-3}

Multiplying out the denominator and simplification yields 4(4x-3) = x(4x-3) + 7x - 3 \Longrightarrow (2x-3)^2 = 0, so x = \frac{3}{2}. Substituting leads to y = \frac{2}{5}, z = \frac{5}{3}, and the product of these three variables is 1.

See also

2000 AMC 12 (ProblemsResources)
Preceded by
Problem 19
Followed by
Problem 21
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Want to learn how to tackle those tough MATHCOUNTS and AMC counting and probability problems? Check out Art of Problem Solving's Introduction to Counting & Probability by David Patrick.
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