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imc

geometry intermediate

Problem

A cubical cake with edge length inches is iced on the sides and the top. It is cut vertically into three pieces as shown in this top view, where is the midpoint of a top edge. The piece whose top is triangle contains cubic inches of cake and square inches of icing. What is ?
problem
(A)
(B)
(C)
(D)
Solution
Introduce a coordinate system where , and . In this coordinate system we have , and the line has the equation . As the line is orthogonal to , it must have the equation for some suitable constant . As this line contains the point , we have . Substituting into , we get , and then . We can note that in is the height from onto , hence its area is , and therefore the answer is .
Final answer
B