Maths Olympiad Prep

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Algebra Difficulty 5.1 AIME, harder Prove it Philippines

Problem:

How many roots has the equation sinxlog10x=0\sin x - \log_{10} x = 0?

Solution

Solution:

(ans. 3 .
sinx=logxx10.10<22π\sin x = \log x \Rightarrow x \leq 10.10 < 2 \cdot 2\pi means that in [0,2π][0, 2\pi] there is a complete period of sin\sin and part of a second period. There is an intersection in the first period, and after the first period, near 5π2\frac{5\pi}{2} to the left and to the right, sin\sin has value <1<1 (and >0>0, of course). Since 2π+π2=5π2<102\pi + \frac{\pi}{2} = \frac{5\pi}{2} < 10, sinx\sin x intersect logx\log x at two points to the left and right side of 5π2\frac{5\pi}{2}.)

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.