The latin letter r is used in math as a variable. It appears in geometric equations as a variable to represent the radius of a circle.
The capital Latin letter R is used in mathematics to represent the set of real numbers. Usually, the letter is presented with a "double-struck" typeface when it is used to represent the set of real numbers.
The circumference of a circle is given the constant τ (tau) multpilied by the radius of the circle, where τ = 2π.
The area of a circle is give by one-half multiplied by τ (tau) mutliplied by the radius of the circle squared.