What is the area of the curved surface of a right circular cone of radius 5 and height 12?

The area is given by

pi * (cone radius) * slant height of cone =

pi * 5 * sqrt (5^2 + 12^2) =

5 * 13 * pi =

65pi units^2 ≈ 204.2 units^2

How can you do a fraction that is involved in the equation

Could you give an example????