The number we need to multiply the radius of a circle by to get the diametre is always 2.
Diametre of circle = 2 × Radius of circle
The number we need to multiply the side of an equilateral triangle by to get the perimeter is always 3.
Perimeter of equilateral triangle = 3 × Length of side
The number we need to multiply the diametre of a circle by to get the circumference is always π (pi)
Circumference of circle = π × Diametre of circle