# A Beautiful Expression for Pi

## Proving a surprising and pleasing mathematical result

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*.*