In a regular pyramid

• Draw the right-angled triangle whose sides are the height h, the distance from the centre of the base to the midpoint of one of its sides (called the apothem of the base, written m), and the slant height l. Then:
• l² = h² + m²

In a right circular cone

1. The right-angled triangle with sides h, r and l gives:
2. l² = h² + r²