Proof that pi is irrational

Have you ever seen the proof that \pi 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.



  1. David Freeman says:

    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:
    “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?

    1. William Romaine says:

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

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