Maths Olympiad Prep

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, 2024

Algebra Difficulty 4.8 AIME Find the answer Canada

For how many integers mm does
the line with the equation y=mxy = mx
intersect the line segment with endpoints (20,24)(20, 24) and (4,202)(4, 202)?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Let OO be the origin, AA be the point with coordinates (20,24)(20,24), and BB be the point with coordinates (4,202)(4, 202).

The slope of OAOA is 2420=1.2\frac{24}{20} = 1.2. The slope of OBOB is 2024=50.5\frac{202}{4} = 50.5.

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The line with equation y=mxy = mx
passes through (0,0)(0,0).

If m<0m < 0, the line with equation
y=mxy = mx does not pass through the
first quadrant.

If 0m<1.20 \leq m < 1.2, the line with
equation y=mxy = mx is less steep than
OAOA and so does not intersect line
segment ABAB.

If m>50.5m > 50.5, the line with
equation y=mxy = mx is steeper than
OBOB and so does not intersect line
segment ABAB.

For integers mm with 2m502 \leq m \leq 50, the line with equation
y=mxy = mx will intersect the line
segment ABAB.

There are 4949 such integers mm.

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Source: CEMC, University of Waterloo, licensed CC-BY-NC-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.