Question 893993
This really works better with a picture.

Top diameter for the tank is 3 m, and distance top to bottom of the tank is 10 m.
The ratio of diameter to height, {{{d/h=3/10}}}.  This is using d for DIAMETER and h for HEIGHT or DEPTH.  You can recognize similar triangles if this helps.


If you want the surface area of the top of the liquid when the filled depth is 5 m, then:


{{{d=h(3/10)}}}
{{{d=5(3/10)}}}
{{{d=3/2}}}
Know that half of diameter is radius; so the radius at that depth is {{{r=d/2=3/2/2=3/4}}}


Now to get the value for this surface area of the liquid at the top of this depth, {{{highlight(pi*(3/4)^2)}}}.