To find the equation of a circle when you know the radius and centre, use the formula (x a) 2 + (y b) 2 = r 2, where (a, b) represents the centre of the circle, and r is the radius.

Press "Enter" to calculate the radius in the same units used for diameter. Enter "=Circumference/ (2*PI ())" without quotes, replacing "Circumference" with its measurement, to calculate radius.