2022 AMC 10A Problem 18

Attempt Problem 18 of the 2022 AMC 10A below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2022 AMC 10A solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

18.

Let TkT_k be the transformation of the coordinate plane that first rotates the plane kk degrees counterclockwise around the origin and then reflects the plane across the yy-axis. What is the least positive integer nn such that performing the sequence of transformations T1,T2,T3,,TnT_1, T_2, T_3, \cdots, T_n returns the point (1,0)(1,0) back to itself?

359359

360360

719719

720720

721721

Answer: A
Concepts:transformationpattern recognition
Difficulty rating: 1950
Solution:

Since we are working with angles and reflections, working with polar coordinates would make this problem easier to deal with.

Let (r,θ)(r, \theta) be a polar coordinate. Rotating this by kk degrees counterclockwise maps the point to (r,θ+k)(r, \theta + k^{\circ}) and then reflecting it maps it to (r,180θk).(r, 180^{\circ} - \theta - k^{\circ}).

Therefore, we have that Tk(r,θ)=(r,180θk). T_k(r, \theta) = (r, 180^{\circ} - \theta - k^{\circ}).

From this, we can see that Tk+1(Tk(r,θ))= T_{k + 1}(T_k(r, \theta)) = Tk+1(r,180θk)=T_{k + 1}(r, 180^{\circ} - \theta - k^{\circ}) = (r,θ1).(r, \theta - 1^{\circ}).

Now, let's analyze what happens to the point (1,0).(1, 0^{\circ}).

After T1,T_1, we get (1,179).(1, 179^{\circ}).

After T2,T_2, we get (1,1).(1, -1^{\circ}).

After T3,T_3, we get (1,178).(1, 178^{\circ}).

After T4,T_4, we get (1,2).(1, -2^{\circ}).

\vdots

After T2m1,T_{2m - 1}, we get (1,180m).(1, 180^{\circ} - m^{\circ}).

After T2m,T_{2m}, we get (1,m).(1, -m^{\circ}).

From this, we can see that the first time the angle is back to 00^{\circ} is after T2(180)1=T359.T_{2(180)-1}=T_{359}. Therefore n=359.n=359.

Thus, A is the correct answer.

← Problem 17#17
Full Exam

Problem 18 in Other Years