Tag Archives: numbers

How to memorize the value of e to 15 decimal places

I learned the following trick for memorizing the value of e from my colleague, Gene White. It never fails to impress calculus students (given a wide enough definition of “impress”).

Start by carefully looking at this picture:

That’s a 20 dollar bill, so memorize “2” and put down the decimal point.

The picture on the bill is of Andrew Jackson. He was our seventh President, so put a “7” after the decimal point to get 2.7.

Jackson was elected in 1828, so put down “1828” next. Since there’s a 2 in front of the decimal place, put “1828” a second time. We’re now up to 2.718281828.

Now look at the red square over Jackson’s face. The diagonal creates two congruent right triangles with angle measures 45, 90, and 45. So, add on 459045 to get 2.718281828459045. And that’s e to 15 places.

I’m open to suggestions on how to memorize more of the digits.

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Filed under Calculus, Geekhood, Math, Teaching

How big is 10 to the 20th?

Here’s a great illustration from George Gamow’s classic book One Two Three… Infinity which shows two things: just how big 10^{20} really is, when thought of as a scaling factor; and also the power of a good illustration to drive home a point about math or science. The picture shows a normal-sized astronomer observing the Milky Way galaxy when shrunk down by a factor of 10^{20}

That’s a big number, folks. 

Gamow’s book is one of several on my summer reading list, and there’s a reason it’s a classic. In particular, it’s chock full of cool illustrations like this that convey more information about a science concept than an hour’s worth of lecturing. 

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Filed under Geekhood, Math, Science