A solid having a circular base and vertex above the centre of circular base is called a regular cone.
A → Vertex
‘O’ centre of ground
OB is the radius of cone (r) and
OA is the height of cone (h)
AB is slant height (l)
Now ΔAOB is a right angled triangle.
Where ∠AOB = 90 and AB is hypotenuse.
Then from Pythagoras theorem
AB2 = OA2 + OB2 = r2 + h2
∴ AB = \(\sqrt{r^2+h^2}\)
∴ Slant height (l) = \(\sqrt{r^2+h^2}\)