A cone resting on its base, with a dashed line from the side of the circle to the center of the circle labeled r. The edge of the cone is labeled Slant height (l).
The surface area of a cone is given by the formula
S = πl + πr2. Solve the formula for l.

l = S – r2
l = S + r2
l = l equals StartFraction S Over pi minus r squared. – r2
l = l equals StartFraction S Over pi plus r squared. + r2