A Beautiful Expression for Pi

Proving a surprising and pleasing mathematical result

Keith McNulty

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Today I am going to prove a beautiful and somewhat surprising expression for π. The expression that I will prove is the following:

where the denominator is an infinite product of nested surds.

Step 1 of my proof will establish an important trigonometric identity, which we will use in Step 2 by taking limits and applying the result to π.

Proving an important trigonometric identity

Our expression for π can be derived from the following trigonometric identity:

for all positive integers n and where ⍺ is not an integer multiple of π.

We are going to use classic induction to prove this. Remember that you can prove a result for all positive integers n if and only if you can prove it is true for n = 1 (the induction start), and you can prove that if it is true for n = k then it is true for n = k + 1 (the induction step).

To prove the induction start, let’s use the standard trigonometric identity sin(θ+ϕ) = sinθcosϕ + cosθsinϕ. If θ = ϕ = ⍺/2, we have sin⍺ = 2sin(⍺/2)cos(⍺/2). Dividing both sides by cos(⍺/2) gives the result for n = 1.

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Keith McNulty

Pure and Applied Mathematician. LinkedIn Top Voice in Tech. Expert and Author in Data Science and Statistics. Find me on LinkedIn, Twitter or keithmcnulty.org