Posted by: **Dave Richeson** | February 11, 2010

## Proof that pi is irrational

Have you ever seen the proof that is irrational? If not, I highly recommend heading over to The Math Less Traveled. Blogger Brent Yorgey just posted the last of his six part series in which he gives Ivan Niven’s easy-to-follow 1947 proof of that famous fact. The proof uses only basic calculus.

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I’m retired and my math chops are pretty rusty.

While trying to understand Niven’s proof that pi is irrational, I decided to back up and refresh my memory on some more basic trigonometry and I stumbled on this statement at: http://www.themathpage.com/aTrig/circle.htm

“It should be intuitively clear that π cannot be a rational number, because it indicates the ratio of a curved line to a straight. And to name such a ratio exactly is impossible.”

This is not at all intuitively obvious to me. In fact, it seems to me that the statement above is obviously incorrect. If I take a straight line and bend it, the ratio of its bent length to its straight length will still be 1.

Am I missing something?

By:

David Freemanon February 12, 2010at 11:29 am

Dear Sir:

By this time, you have probably had several replies. When a line (segment) is bent, its shape changes. Tape-measures are useful.

By:

William Romaineon June 12, 2012at 10:10 am