Tanya Khovanova's Math Blog

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—What is the best way to pass a geometry test?
—Know all the angles.

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—Did you hear about the over-educated circle?
—It has 360 degrees!

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—What do parallel lines and vegetarians have in common?
—They never meat.

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—Did you hear about the mathematician who’s afraid of negative numbers?
—He’ll stop at nothing to avoid them.

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—What do you call a gentleman who spent all the summer at the beach?
—A tangent.

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—What do mathematicians and the Air Force have in common?
—They both use pi-lots.

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—Why can’t a nose be 12 inches long?
—Because then it would be a foot.

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—Are monsters good at math?
—Not unless you Count Dracula.

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—Why did the math professor divide sine by tan?
—Just cos.

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Two is the oddest prime.

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There are a lot of puzzles where you need to guess something asking only yes-or-no questions. In this puzzle, there are not two but three possible answers.

Puzzle. Mike thought of one of three numbers: 1, 2, or 3. He is allowed to answer “Yes”, “No”, or “I don’t know”. Can Pete guess the number in one question?

Yes, he can. This problem was in one of my homeworks, and my students had a lot of ideas. Here is the first list were ideas are similar to each other.

• I am thinking of an odd number. Is my number divisible by your number?
• If I were to choose 1 or 2, would your number be bigger than mine?
• If I were to pick a number from the set {1,2,3} that is different from yours, would my number be greater than yours?
• If I have a machine that takes numbers and does nothing to them except have a 50 percent chance of changing a two to a one. Would your number, after going through the machine, be one?
• If I were to choose a number between 1.5 and 2.5, would my number be greater than yours?
• If your number is x and I flip a fair coin x times, will there be at least two times when I flip the same thing?
• I am thinking of a comparison operation that is either “greater” or “greater or equal”. Does your number compare in this way to two?

One student was straightforward.

• Mike, please, do me a favor by responding ‘yes’ to this question if you are thinking about 1, ‘no’ if you are thinking about 2, and ‘I don’t know’ if you are thinking about 3?

One student used a famous unsolved problem: It is not known whether an odd perfect number exists.

• Is every perfect number divisible by your number?

Then, I gave this to my grandchildren, and they decided to answer in a form of a puzzle. Payback time.

• I’m thinking of a number too, and I don’t know whether it’s double yours. Is the sum of our numbers prime?

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I rarely post physics puzzles, but this one is too good to pass on.

Puzzle. A wireframe icosahedron is assembled so that each of its edges has a resistance of 1. What is the total resistance between opposite vertices of the icosahedron?

While we are at it, another interesting question would be the following.

Puzzle. A wireframe cube is assembled so that each of its edges has a resistance of 1. What is the total resistance between opposite vertices of the cube?

And this reminds me of a question I heard when I was preparing for an IMO many years ago.

Puzzle. A wireframe infinite square grid is assembled so that each of its edges has a resistance of 1. What is the total resistance between two neighboring vertices?

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Have you ever heard of ikigai? The Japanese concept which gives four simple requirements for a happy career:

• Do what you love.
• Do what you are good at.
• Do what impacts the world.
• Do what you can get paid for.

I often think about it for myself and for my students. Is this good advice for finding a career path?

I like that ikigai separates the first two requirements: passion and gift. Many of my students do not see the difference, as passion and gift are highly correlated. When you love something, you practice it and become better at it. When you are good at something, it becomes easy and enjoyable.

Nonetheless, passion and gift are different. Unfortunately, I’ve seen students who are good at math only because their parents push them, but they do not love it. Some of them already found their passion but are afraid to tell their parents. Some haven’t yet found their passion, but it is perfectly clear that math is not it. So, a gift doesn’t imply passion.

What about the other way around? My programs are too selective, so I haven’t seen students who are not gifted in math. I will use myself as an example. I have always passionately loved dancing, but it is obvious that my dancing career would have been a disaster. I am very happy I closed that career path in fifth grade.

Anyway, the first two ikigai requirements are not the same, and both are necessary.

The third ikigai requirement is about doing what the world needs. Impacting the world is a great motivator and makes you feel good. And yet, I see happy and successful mathematicians who only care about the beauty of what they are doing and nothing else. This requirement is important but might not be a deal breaker for everyone.

The last ikigai requirement is crucial. If you are not being paid for your efforts, it is not a career; it is a hobby. I got attracted to it because it includes an important caveat: you need to find people who want to pay you for what you can offer. I recently wrote an essay Follow Your Heart? about many young aspiring opera singers who ignored this last requirement and ended up changing careers.

Nevertheless, the whole concept of ikigai bugs me. People who find their dream job might agree to work for much less pay than they are worth. It opens them up to potential exploitation by greedy employers.

Have I reached my ikigai? Judging by my low pay, I am close.

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