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Algebra Difficulty 4.0 AMC 10/12 Find the answer

Given the function f(x)f(x) with the domain [1,+)[1, +\infty), and f(x)={12x3,1x<212f(12x),x2f(x) = \begin{cases} 1-|2x-3|, & 1\leq x<2 \\ \frac{1}{2}f\left(\frac{1}{2}x\right), & x\geq 2 \end{cases}, then the number of zeros of the function y=2xf(x)3y=2xf(x)-3 in the interval (1,2017)(1, 2017) is \_\_\_\_\_\_.

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

Solution

Let the function y=2xf(x)3=0y=2xf(x)-3=0, which leads to the equation f(x)=32xf(x) = \frac{3}{2x},

- When x[1,2)x \in [1, 2), the function f(x)f(x) first increases and then decreases, reaching its maximum value of 1 at x=32x = \frac{3}{2},
and y=32xy = \frac{3}{2x} also has y=1y=1 at x=32x = \frac{3}{2};

- When x[2,22)x \in [2, 2^2), f(x)=12f(12x)f(x) = \frac{1}{2}f\left(\frac{1}{2}x\right), and at x=3x=3, the function f(x)f(x) reaches its maximum value 12\frac{1}{2},
and y=32xy = \frac{3}{2x} also has y=12y= \frac{1}{2} at x=3x=3;

- When x[22,23)x \in [2^2, 2^3), f(x)=12f(12x)f(x) = \frac{1}{2}f\left(\frac{1}{2}x\right), and at x=6x=6, the function f(x)f(x) reaches its maximum value 14\frac{1}{4},
and y=32xy = \frac{3}{2x} also has y=14y= \frac{1}{4} at x=6x=6;

- ...;

- When x[210,211)x \in [2^{10}, 2^{11}), f(x)=12f(12x)f(x) = \frac{1}{2}f\left(\frac{1}{2}x\right), and at x=1536x=1536, the function f(x)f(x) reaches its maximum value 1210\frac{1}{2^{10}},
and y=32xy = \frac{3}{2x} also has y=1210y= \frac{1}{2^{10}} at x=1536x=1536.

Therefore, the number of zeros of the function y=2xf(x)3y=2xf(x)-3 in the interval (1,2017)(1, 2017) is 11\boxed{11}.

By setting the function y=2xf(x)3=0y=2xf(x)-3=0, we obtain the equation f(x)=32xf(x) = \frac{3}{2x}, thereby transforming the problem of finding the zeros of the function into finding the roots of the equation, which is further transformed into a problem of finding the intersection points of two functions, and then the answer is obtained by discussing each interval separately.

This problem examines the relationship between the zeros of a function and the roots of an equation, as well as the application of the intersection points of functions, reflecting the mathematical transformation thought method and the mathematical thought method of classified discussion, making it a challenging problem.

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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.