1. cut the sphere into 2 and u see a circle.

2. put that circle on a graph paper, such that the y-axis is the depth of water, and the x-axis is the radius of the cross section.

3. you end up with a circle of origin (0,r_{o})

4. the general formula of a circle is (x - a)^{2} + (y - b)^{2} = R^{2} where (a,b) is the centre and R is the radius of the circle.

5. Hence, I use the formula (h - r_{o})^{2} + r^{2} = r_{o}^{2}