GeometryDifficulty 3.8AMC 10/12Find the answerCanada
A robot is programmed to complete the following sequence of steps:
Step 1: Move 2 m in the direction it is facing. Step 2: Turn 90° to the left. Step 3: Move 4 m in the direction it is facing.
The robot starts facing north and completes this sequence of 3 steps a total of 26 times. When it has completed these steps, the robot is $x m} from its starting point. What is the value of x^2$?
A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.
Solution
Consider placing the robot’s movement onto the xy plane.
Suppose that its starting point is O(0,0), north is in the positive y direction, and 1 unit in the plane represents 1 m travelled by the robot.
The robot begins facing north and so in Step 1 it travels 2 m to the point A(0,2), as shown.
After turning 90° left (to face west) and moving 4 m in the direction that it is facing (Step 2 and Step 3), the robot is at B(−4,2).
The robot then repeats these 3 steps, moving 2 m forward to C(−6,2), turning 90° (to face south), and moving 4 m in the direction that it is facing to D(−6,−2).
Repeating these 3 steps a 3rd time, the robot moves to E(−6,−4) and then to F(−2,−4).
Repeating these 3 steps a 4th time, the robot moves to G(0,−4) and then to its starting point O(0,0), and is facing north.
[[IMAGE0]]
Therefore, each time the robot completes this sequence of 3 steps a total of 4 times, it ends at O(0,0) and is facing north.
Thus, if the robot completes this sequence of 3 steps 4×6=24 times, it ends at (0,0) and is facing north.
Completing the sequence of 3 steps a 25th time, the robot moves to B(−4,2) and is facing west (facing point C).
Finally after completing the sequence a 26th time, the robot ends at D(−6,−2), and so its distance from its starting point O(0,0) is x=(0−(−6))2+(0−(−2))2=36+4=40, and so x2=40.
Want a route through all this instead of an archive? The track
puts 2,444 problems in a working order, from Junior Challenge level to the IMO shortlist.