#[http://geogebratube.org/material/show/id/144079 Geogebra file] for explaining how 'circumference / diameter' is a constant, denoted as pi (Greek letter), using a number line | #[http://geogebratube.org/material/show/id/144079 Geogebra file] for explaining how 'circumference / diameter' is a constant, denoted as pi (Greek letter), using a number line |