# Day 10 - the glorious tau

Relevancy: 1.9 stable

τ (tau) is one of the most important mathematical constants (if not the most important), relating circle's circumference to it's radius. See the tau manifesto for the long explanation if you're still an unbeliever. Can we use it in Rust? Of course, there's a crate for that!

Let's see this amazing number in full glory:

day10.rs

``````extern crate tau;
``````

day10.rs

``````println!("τ = {}", tau::TAU);
``````
``````\$ cargo run
τ = 6.283185307179586
``````

We can do all sorts of important calculations with τ, just look:

day10.rs

``````let radius: f64 = 15.0;
println!("circle circumference = τ * r = {}", tau::TAU * radius);
let c: Complex64 = Complex::from_polar(&1.0, &tau::TAU);
println!("Euler's identity: exp(i * τ) = {}", c);
println!("Trigonometry: sin(τ) = {}, cos(τ) = {}", tau::TAU.sin(), tau::TAU.cos());
``````

And if someone really, I mean really needs to refer to that other mathematical constant, it is (regrettably) possible as well.

day10.rs

``````println!("That other constant = {}", tau::TAU / 2.0);
``````

Here's the output:

``````\$ cargo run
circle circumference = τ * r = 94.24777960769379
Euler's identity: exp(i * τ) = 1-0.00000000000000024492935982947064i
Trigonometry: sin(τ) = -0.00000000000000024492935982947064, cos(τ) = 1
That other constant = 3.141592653589793
``````