Maths Olympiad Prep

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Geometry Difficulty 3.8 AMC 10/12 Find the answer Canada

A robot is programmed to complete the following sequence of
steps:

Step 1: Move 2 m\text{2 m} in
the direction it is facing.
Step 2: Turn 90°90\degree to
the left.
Step 3: Move 4 m\text{4 m} in
the direction it is facing.

The robot starts facing north and completes this sequence of 33 steps a total of 26 times. When it has
completed these steps, the robot is $x \text{}
m} from its starting point. What is the value of x^2$?

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 xyxy plane.

Suppose that its starting point is O(0,0)O(0,0), north is in the positive yy direction, and 1 unit in the plane
represents 11 m travelled by the
robot.

The robot begins facing north and so in Step 1 it travels 22 m to the point A(0,2)A(0,2), as shown.

After turning 90°\degree left (to
face west) and moving 44 m in the
direction that it is facing (Step 2 and Step 3), the robot is at B(4,2)B(-4,2).

The robot then repeats these 3 steps, moving 22 m forward to C(6,2)C(-6,2), turning 90°\degree (to face south), and moving 44 m in the direction that it is facing to
D(6,2)D(-6,-2).

Repeating these 3 steps a 3rd time, the robot moves to E(6,4)E(-6,-4) and then to F(2,4)F(-2,-4).

Repeating these 3 steps a 4th time, the robot moves to G(0,4)G(0,-4) and then to its starting point
O(0,0)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)O(0,0)
and is facing north.

Thus, if the robot completes this sequence of 3 steps 4×6=244\times6=24 times, it ends at (0,0)(0,0) and is facing north.

Completing the sequence of 3 steps a 25th time, the robot moves to B(4,2)B(-4,2) and is facing west (facing point
CC).

Finally after completing the sequence a 26th time, the robot ends at
D(6,2)D(-6,-2), and so its distance from
its starting point O(0,0)O(0,0) is x=(0(6))2+(0(2))2=36+4=40x=\sqrt{(0-(-6))^2+(0-(-2))^2}=\sqrt{36+4}=\sqrt{40},
and so x2=40x^2=40.

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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.