This is another problem from the indefatigable Presh Talwalkar.

_ _____Hard Geometry Problem

“In triangle ABC above, angle A is bisected into two 60° angles. If AD = 100, and AB = 2(AC), what is the length of BC?”

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This is another problem from the indefatigable Presh Talwalkar.

_ _____Hard Geometry Problem

“In triangle ABC above, angle A is bisected into two 60° angles. If AD = 100, and AB = 2(AC), what is the length of BC?”

Having fallen under the spell of Catriona Shearer’s geometric puzzles again, I thought I would present the latest group assembled by Ben Orlin, which he dubs “Felt Tip Geometry”, along with a bonus of two more recent ones that caught my fancy as being fine examples of Shearer’s laconic style. Orlin added his own names to the four he assembled and I added names to my two, again ordered from easier to harder.

See Geometric Puzzle Munificence.

**(Update 4/16/2020)** Ben Orlin has another set of Catriona Shearer puzzles 11 Geometry Puzzles That Drive Mathematicians to Madness which I will leave you to see and enjoy. But I wanted to emphasize some observations he included that I think are spot on. Continue reading

This is a surprisingly challenging puzzle from the *Mathematics 2020* calendar.

“The sketch is of equally spaced railroad ties drawn in a one point perspective. Two of the ties are perceived to the eye to be 25 feet and 20 feet respectively. What is the perceived length x of the third tie?”

Even though the ties are equally-spaced and of equal length in reality, from the point of view of perspective they are successively closer together and diminishing in length. The trick is to figure out what that compression factor is. I had to review my post on the Perspective Map to get some clues.

See the Railroad Tie Problem

The following problem from *Five Hundred Mathematical Challenges* was a challenge indeed, even though it appeared to be a standard travel puzzle.

“**Problem 118**. Andy leaves at noon and drives at constant speed back and forth from town A to town B. Bob also leaves at noon, driving at 40 km per hour back and forth from town B to town A on the same highway as Andy. Andy arrives at town B twenty minutes after first passing Bob, whereas Bob arrives at town A forty-five minutes after first passing Andy. At what time do Any and Bob pass each other for the nth time?”

See the Perpetual Meetings Problem

This is a problem from a while back (2015) at *Futility Closet*.

“Which part of this square has the greater area, the black part or the gray part?”

See Modern Art

Here is another engaging problem from Presh Talwalkar.

“___________**Triangle Area 1984 AIME**

Point P is in the interior of triangle ABC, and the lines through P are parallel to the sides of ABC. The three triangles shown in the diagram have areas of 4, 9, and 49. What is the area of triangle ABC?”

See the Pinwheel Area Problem

Here is another challenging problem from the 2004 *Pi in the Sky* Canadian magazine for high school students.

“**Problem 4**. Find the real solutions of the system

________________ (x + y)^5 = z,

________________ (y + z)^5 = x,

________________ (z + x)^5 = y.”

See the Quintic Nightmare

This is another fairly simple puzzle from *Futility Closet.*

“If an equilateral triangle is inscribed in a circle, then the distance from any point on the circle to the triangle’s farthest vertex is equal to the sum of its distances to the two nearer vertices (q = p + r).

(A corollary of Ptolemy’s theorem.)”

See A Tidy Theorem

Here is another simply amazing problem from *Five Hundred Mathematical Challenges*:

“**Problem 154**. Show that three solutions, (x1,.y1), (x2,.y2), (x3, y3), of the four solutions of the simultaneous equations

____________(x – h)² + (y – k)² = 4(h² + k²)

______________________xy = hk

are vertices of an equilateral triangle. Give a geometrical interpretation.”

Again, I don’t see how anyone could have discovered this property involving a circle, a hyperbola, and an equilateral triangle. It seems plausible when h.=.k, but it is not at all obvious for h.≠.k. For some reason, I had difficulty getting a start on a solution, until the obvious approach dawned on me. I don’t know why it took me so long.

See the Amazing Triangle Problem.

Here is another train puzzle, this time from J. A. H. Hunter’s *Entertaining Mathematical Teasers*:

“Mike had made the [train] trip many times. ‘That’s the morning express from Tulla we’re passing,’ he said. ‘It left Tulla one hour after we pulled out from Brent, but we’re just 25% faster.’ ‘That’s right, and we’re also passing Cove, two-thirds the distance between Brent and Tulla,’ Martin agreed. ‘So we’re both right on schedule.’ Obviously a couple of train buffs! Assuming constant speeds and no stops, how long would it be before they reached Tulla?”

See the Train Buffs