So here is the problem:
A baking pan has a rectangular base 12 inches by 8 inches; the sides and ends of the pan slope outward, so that the upper edges measure respectively 13.5 inches by 9 inches. If the depth of the pan is 2 inches, find the amount of cake batter required to fill the pan to one-half its depth.
I answered the problem and here is my solution and apologize for my bad drawing and bad grammar:
Pyramid Frustrum and Its Dimensions
But I wanted to solve the problem with different approach, in here I applied this formula from similar figures:
Then I noticed if I calculate the base if the height is half, then they do not have the same value:
Is the formula that I am using is invalid or I am using it wrong? Thanks!