Skip to content

Quod erat demonstrandum

QED 1

Some time ago, I went through a proof of the quadratic formula with the pre-AP kids. It was part of our notes. I put the proof in their homework. The proof was an extra credit question on their test. At the end of the proof, when we had arrived at our destination, I wrote on the board: QED.

I turned to the kids.

Quod erat demonstrandum,” I said. I rolled the r’s. I flattened the vowel sounds. I did my best to sound exotic.

“What’s that!?” a student asked.

“That which was to be shown,” I said. “It’s latin”.

And I repeated it again.

QED 2

We did another proof a few weeks ago. Frankly these proofs are not so much proofs per se as derivations. So yeah. We derived something else. And when we got to the end, I wrote: QED.

One of the students said, “Tell us what that stands for again, Mr. Hasan.”

Of course, they knew what it stood for. What they were asking for was the latin. They wanted to hear it.

Quod erat demonstrandum,” I said. I rolled the r’s. I flattened the vowel sounds. I did my best to sound exotic.

Professor Langebartel

When I was an undergraduate at the University of Illinois, I took a Differential Equations and Orthogonal Functions course from Dr. Langebartel. He was awesome, so awesome that when I needed a math elective to finish my minor I chose Tensor Analysis simply because he taught it.

Dr. Langebartel once stopped a lecture and asked us, “Who was the greatest mathematician in history?” He waited briefly, but this was a rhetorical question. He answered it himself.

“Eudoxus,” he said.

We were in an auditorium in Altgeld Hall (which is going to get a facelift soon). I can still hear his booming voice in that large hall drawing out his sibilantsEudoxxxxxusssss! And he told us a story about Eudoxus.

It was a surprise to have a professor give historical context in a technical course. No other professors I had did this. He was the one from whom I first learned QED. And I still remember how exotic it sounded for him to draw out the latin pronunciation. Clearly my students feel the same way.

My students thank you for those lessons, professor.