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jmc

geometry senior

Problem

In right triangle , , , and . Points and are midpoints of and respectively; and intersect at point . Compute the ratio of the area of quadrilateral to the area of triangle .
Solution
We begin by drawing a diagram:

Since and are midpoints, and are medians. Let be the midpoint of ; we draw median . The medians of a triangle are always concurrent (pass through the same point), so passes through as well.



The three medians cut triangle into six smaller triangles. These six smaller triangles all have the same area. (To see why, look at and notice that and have the same area since they share an altitude and have equal base lengths, and and have the same area for the same reason. Thus, and have the same area. We can repeat this argument with all three sizes of triangles built off the other two sides and , to see that the six small triangles must all have the same area.)

Quadrilateral is made up of two of these small triangles and triangle is made up of two of these small triangles as well. Hence they have the same area (and this will hold true no matter what type of triangle is). Thus, the ratio of the area of quadrilateral to the area of triangle is .
Final answer
1