Figure III is the difference between an isosceles triangle with sides , and and an isosceles triangle with sides , and .
The area is cm2.
The circumference is cm.
This is the case when the area of the trapezium that defines the front of the container
is divided into two equal parts. The area of the trapezium is cm2.
If the height of the lower half is , you can show by using similarity that the longest of the two parallel sides if this trapezium has a lenght of . The area of this lower trapezium is half the area of the total trapezium, so .
This gives or: .
From this you find cm.
The area of the water surface is approximately cm2.