Slant height

The slant height of a right circular cone is the distance from any point on the circle to the apex of the cone.

The slant height of a cone is given by the formula $\sqrt{r^2+h^2}$, where r is the radius of the circle and h is the height of a square

It is trivial to see why this formula holds true. If a right triangle is inscribed inside the cone, with one leg of the triangle being the line segment from the center of the circle to its radius, and the second leg of the triangle being from the apex of the cone to the center of the circle, then one leg will have length h, another leg will have length r, and by the Pythagorean theorem, r² + h² = d², and $d=\sqrt{r^2+h^2}$ gives the length of the circle to the apex of the cone.