Here is yet another (belated) collection of beautiful geometric problems from Catriona Agg (née Shearer).

# Tag Archives: plane geometry

# Moon Quarters Problem

This is a straight-forward problem from the Scottish Mathematical Council (SMC) Senior Mathematics Challenge.

“A circle has radius 1 cm and *AB* is a diameter. Two circular arcs of equal radius are drawn with centres *A* and *B*. These arcs meet on the circle as shown. Calculate the shaded area.”

There are several possible approaches and the SMC offers two examples.

See the Moon Quarters Problem

# Max Angle Puzzle

Here is a familiar puzzle from the *Mathigon* Puzzle Calendars for 2021.

“Given a line and two points A and B, which point P on the line forms the largest angle APB?”

See the Max Angle Puzzle

An excellent application of the solution to this puzzle can be found at Numberphile, where Ben Sparks explains an optimal rugby goal-kicking strategy.

**(Update 3/23/2023) Solution Construction**

# Line Work

This is a fairly simple problem from Futility Closet, which is currently under a hiatus.

“Robert Bilinski proposed this problem in the April 2006 issue of *Crux Mathematicorum*. On square *ABCD*, two equilateral triangles are constructed, *ABE* internally and *BCF* externally, as shown. Prove that *D*, *E*, and *F* are collinear.”

See Line Work

# Covering Rectangle Puzzle

This is a nice puzzle from the Maths Masters team, Burkard Polster (aka Mathologer) and Marty Ross as part of their “Summer Quizzes” offerings.

“In the picture does the green rectangle cover more or less than half of the [congruent] red rectangle?”

It is evident from the problem solution that the two rectangles are the same, so I made it explicit.

See the Covering Rectangle Puzzle

# Linked Triangles Problem

I found this problem from the 1981 Canadian Math Society’s magazine, *Crux Mathematicorum*, to be quite challenging.

“*Proposed by Kaidy Tan, Fukien Teachers’ University, Foochow, Fukien, China.*

An isosceles triangle has vertex A and base BC. Through a point F on AB, a perpendicular to AB is drawn to meet AC in E and BC produced in D. Prove synthetically that

Area of AFE = 2 Area of CDE if and only if AF = CD.”

See the Linked Triangles Problem

**(Update 2/22/2023, 6/9/2023) Alternative Solutions**

# Rolling Wheels Puzzle

Here is another *Quantum* math magazine Brainteaser.

“Two wheels roll toward each other with identical angular velocity. At the moment of collision they contact each other at the same points that touched the ground before they began rolling. Could the radii of the wheels differ?”

See the Rolling Wheels Puzzle

# Three Triangles Puzzle

This is a nice little puzzle from the late Nick Berry’s Datagenetics Blog.

“A quick little puzzle this week. (I tried to track down the original source, but reached a dead-end with a web search as the site that hosted it, a blogspot page under the name *fivetriangles* appears password protected, and no longer maintained). …

There are three identical triangles with aligned bases (in the original problem, it is stated they are equilateral, but I don’t think that really matters; Any congruent triangles will do, and I’m going to use isosceles triangles in my solving). If we say that one triangle has the area A, what is the area of the two shaded regions?”

See the Three Triangles Puzzle.

# Spiral Areas Puzzle

This is a provocative puzzle from the Maths Masters team, Burkard Polster (aka Mathologer) and Marty Ross as part of their “Summer Quizzes” offerings for 2013.

“In the picture the top curve is a semicircle and the bottom curve is a quarter circle. Which has greater area, the red square or the blue rectangle?”

See the Spiral Areas Puzzle

# Wisdom of Old

Here is another Brainteaser from the *Quantum* magazine.

“King Arthur ordered a pattern for his quarter-circle shield. He wanted it to be painted in three colors: yellow, the color of kindness; red, the color of courage: and blue the color of wisdom. When the artist brought in his work, the king’s armor-bearer said there was more courage than wisdom on the shield. But the artist managed to prove that the proportions of both virtues were equal. Can you tell how? (A. Savin)”

This is another relatively simple problem, though it may look a bit daunting at first.

See Wisdom of Old