When calculating the surface area of a cone it is necessary to know the length of the slant height. Sometimes the slant height is not given. Because the slant height is also the hypotenuse of a right triangle (the circle's radius and cone's height are the legs), use the Pythagorean Theorem to find the length of the slant height. Watch the following tutorial to see why knowing the Pythagorean Theorem is necessary to find an unknown slant height of a cone.