What If You Only Have a Compass and an Unmarked Ruler? Discover How to Precisely Find the Midpoint of a Line!

Renda Zhang
5 min readApr 10, 2024

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Imagine yourself sitting at a table, in front of you are a few sheets of paper, a pen, an unmarked ruler, and a compass. You’re tasked with a question that seems simple yet intriguing: Without modern measuring tools at your disposal, how would you accurately find the midpoint of a line segment? This is not just a trivial question, but a challenge that takes us back to the fundamentals of mathematics and geometry, employing the most basic tools to solve problems.

In our digital age, we’ve grown accustomed to relying on a variety of high-tech tools to address our questions, from calculators on our smartphones to sophisticated software programs. But if we limit ourselves to using only paper, a pen, an unmarked ruler, and a compass, how do we fare? This scenario might feel like a throwback to geometry class, except this time, we’re without the convenience of modern tools or technology.

This task not only tests our grasp of geometric fundamentals but challenges our ability to think creatively and solve problems using these simple tools — paper, a pen, an unmarked ruler, and a compass. It showcases that, even in a technologically advanced world, traditional methods still hold unique value and beauty.

By exploring how to find the midpoint of a line segment using only these basic tools, we’re not just revisiting and reinforcing our knowledge of geometry. We’re also exercising our logical thinking and problem-solving skills. It might surprise you to find that the solution to a seemingly complex problem often lies in the simplest methods.

So, where do we start? First, we need to gather our tools: paper, a pen, an unmarked ruler, and a compass. These seemingly simple instruments will be our keys to unlocking the mysteries of geometry and revealing the secrets of the midpoint of a line segment.

Now that we have our tools ready, let’s unlock this geometric puzzle step by step and find the midpoint of any line segment.

Step One: Draw the Line Segment

First, draw the line segment you wish to find the midpoint of on a sheet of paper. Let’s call this segment AB for simplicity. This is where our problem-solving journey begins, with the most basic step.

Step Two: Drawing Arcs with the Compass

Next, place one leg of your compass on point A, and adjust the other leg to a length slightly more than half of segment AB. This doesn’t need to be measured exactly — just use your eye to estimate. Then, draw an arc on the paper. It’s important that this arc is large enough to ensure it will intersect with another arc drawn from point B above or below the line segment AB.

Keeping the compass at the same width, move the compass to point B and draw a second arc. You’ll notice that the two arcs intersect above and below line segment AB, forming two intersection points.

Step Three: Mark and Connect the Intersection Points

Now, mark the intersection points of the two arcs on the paper. Let’s call the upper intersection point C and the lower one D. These two points mark a new, imaginary line segment, which will be key to finding the midpoint of AB.

Use your unmarked ruler to connect points C and D by drawing a straight line. This line will intersect line segment AB and, magically, the point where it intersects, let’s call it point M, is the exact midpoint of line segment AB.

Congratulations on Your Achievement

Just like that, without any modern measuring tools, using only paper, a pen, an unmarked ruler, and a compass, we’ve precisely found the midpoint of line segment AB. This process not only demonstrates the wonder and practicality of geometry but also reminds us that even the simplest tools, when used correctly, can accomplish seemingly complex tasks.

Through this activity, we’ve not just revisited the basics of geometry, but we’ve also honed our thinking and problem-solving skills. More importantly, it teaches us that sometimes, when facing challenges, returning to the basics and employing the simplest methods is often the best way to find solutions.

This completes our journey of using paper, a pen, a compass, and an unmarked ruler to find the midpoint of a line segment. Next time you’re faced with a similar geometric problem, consider reaching for these basic tools to find a solution. You might be pleasantly surprised to discover that they can help you see and understand the world around you from a new perspective.

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Renda Zhang
Renda Zhang

Written by Renda Zhang

A Software Developer with a passion for Mathematics and Artificial Intelligence.

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