Maths Olympiad Prep

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, 2012

Algebra Difficulty 5.2 AIME, harder Prove it United States

Problem:

Let nn be the 200th smallest positive real solution to the equation xπ2=tanxx-\frac{\pi}{2}=\tan x. Find the greatest integer that does not exceed n2\frac{n}{2}.

Solution

Solution:

Drawing the graphs of the functions y=xπ2y=x-\frac{\pi}{2} and y=tanxy=\tan x, we may observe that the graphs intersect exactly once in each of the intervals ((2k1)π2,(2k+1)π2)\left(\frac{(2 k-1) \pi}{2}, \frac{(2 k+1) \pi}{2}\right) for each k=1,2,k=1,2, \cdots. Hence, the 200th intersection has xx in the range (399π2,401π2)\left(\frac{399 \pi}{2}, \frac{401 \pi}{2}\right). At this intersection, y=xπ2y=x-\frac{\pi}{2} is large, and thus, the intersection will be slightly less than 401π2\frac{401 \pi}{2}. We have that 401π4=100π+π4=314.16+π4=314\left\lfloor\frac{401 \pi}{4}\right\rfloor=\left\lfloor 100 \pi+\frac{\pi}{4}\right\rfloor=\left\lfloor 314.16+\frac{\pi}{4}\right\rfloor=314.

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