The radius, r, of a circle is one-half of the length of its diameter, d.

How do I write this in paper?

If you mean how you write the equation for finding radius, there are a couple ways.

r = d/2

d/2 = r

r = d • 1/2

d • 1/2 = r

r = d • 0.5

d • 0.5 = r

In the end they all mean either half of d = r or r = half of d.