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Algebra Difficulty 4.1 AIME Find the answer

Let the maximum number among the real numbers x1,x2,,xnx_1, x_2, \ldots, x_n be max{x1,x2,,xn}\max\{x_1, x_2, \ldots, x_n\}, and the minimum number be min{x1,x2,,xn}\min\{x_1, x_2, \ldots, x_n\}. Given real numbers 1xy1 \leq x \leq y and that the three numbers can form the sides of a triangle, if t=max{1x,xy,y}min{1x,xy,y}t = \max\left\{ \frac{1}{x}, \frac{x}{y}, y \right\} \cdot \min\left\{ \frac{1}{x}, \frac{x}{y}, y \right\}, then the range of tt is \_\_\_\_\_\_.

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

Solution

It is obvious that max{1x,xy,y}=y\max\left\{ \frac{1}{x}, \frac{x}{y}, y \right\} = y, and min{1x,xy,y}={1x,if y<x2xy,if yx2\min\left\{ \frac{1}{x}, \frac{x}{y}, y \right\} = \begin{cases} \frac{1}{x}, & \text{if } y < x^2 \\ \frac{x}{y}, & \text{if } y \geq x^2 \end{cases}.

1. When y<x2y < x^2, we have t=yxt = \frac{y}{x}. The feasible region is defined by {1xyy<x+1y<x2\begin{cases} 1 \leq x \leq y \\ y < x + 1 \\ y < x^2 \end{cases}. Since the parabola y=x2y = x^2 intersects the lines y=xy = x and y=x+1y = x + 1 within the first quadrant at the points (1,1)(1, 1) and (1+52,3+52)\left( \frac{1 + \sqrt{5}}{2}, \frac{3 + \sqrt{5}}{2} \right) respectively, we have 1<t<1+521 < t < \frac{1 + \sqrt{5}}{2}.

2. When yx2y \geq x^2, t=xt = x. The feasible region is defined by {1xyy<x+1yx2\begin{cases} 1 \leq x \leq y \\ y < x + 1 \\ y \geq x^2 \end{cases}. Since the parabola y=x2y = x^2 intersects the lines y=xy = x and y=x+1y = x + 1 within the first quadrant at the points (1,1)(1, 1) and (1+52,3+52)\left( \frac{1 + \sqrt{5}}{2}, \frac{3 + \sqrt{5}}{2} \right) respectively, we have 1t<1+521 \leq t < \frac{1 + \sqrt{5}}{2}.

In summary, the range of tt is [1,1+52)\boxed{[1, \frac{1 + \sqrt{5}}{2})}.

This problem examines the range of tt, involves knowledge of parabolas, and requires defining a new feasible region and finding the coordinates of the intersection points within the first quadrant as the key steps.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.