circles

$\tau = 2\pi = 2 * \pi$

$\pi = \tau/2$

1 * Degree = 1/360 * full circle

1 * Minute = 1/60 * Degree

1 * Second = 1/60 * Minute = 1/3600 * Degree

$\pi/2 * Rad = 1/4 * fullcircle$

$\pi Rad = 1/2 * fullcircle$

$2\pi Rad = fullcircle$

$2\pi r = \tau r = Circumfrence = C$

$\pi r^2 = \tau/2 r^2 = Area = A$