Question

Let $T_k$ be the transformation of the coordinate plane that first rotates the plane $k$ degrees counterclockwise around the origin and then reflects the plane across the $y$ -axis. What is the least positive integer $n$ such that performing the sequence of transformations $T_1,$ $T_2,$ $T_3,$ $\ldots,$ $T_n$ returns the point $(1,0)$ back to itself?

$359$

$360$

$719$

$720$

$721$

Answer: A

Concepts:transformation casework

Difficulty rating: 2010

Small Hint:

A point at angle $\theta$ is sent by $T_k$ to angle $(180-k)-\theta$

Big Hint:

Track the angle of $(1,0);$ also consider odd $n,$ where the net map is a reflection

Solution:

Rotating a point at angle $\theta$ by $k^\circ$ gives $\theta+k,$ and reflecting across the $y$ -axis sends angle $\phi$ to $180-\phi.$ So $T_k$ sends $\theta$ to $(180-k)-\theta.$

Starting from $(1,0)$ at angle $0,$ applying $T_1,T_2,\ldots$ gives angles $179,-1,178,-2,177,\ldots.$ After an even number $2m$ of steps the angle is $-m,$ and after an odd number $2m+1$ it is $179-m.$

For the point to return, the angle must be a multiple of $360^\circ.$ The even case needs $m=360,$ i.e. $n=720.$ The odd case needs $179-m=0,$ i.e. $m=179$ and $n=359,$ where the net reflection fixes $(1,0).$

The least such $n$ is $359.$

Thus, the correct answer is A.

2022 AMC 12A Problem 18

Answer: A

Difficulty rating: 2010

Small Hint:

Big Hint:

Solution:

Problem 18 in Other Years