Perpendicular Medians

Geometry Level 4

In the triangle A B C ABC , point D D is located on edge A C AC and point E E is located on edge A B AB . B D BD and C E CE are the medians, and intersect at a right angle at O O . Given B D = 8 BD = 8 and C E = 12 CE = 12 , if x x is the area of triangle A B C ABC , find the value of x + 8 x+8 .


The answer is 72.

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2 solutions

Eloy Machado
Mar 23, 2014

Is given C E CE and B D BD are medians, so O O is the centroid.

Then, C O = 2 3 C E = 8 CO=\frac { 2 }{ 3 } CE = 8 and B O = 2 3 C D = 16 3 BO=\frac { 2 }{ 3 } CD = \frac { 16 }{ 3 } .

Also, triangle B O C BOC is right and its area is [ B O C ] = 1 2 8 16 3 = 64 3 [BOC]=\frac { 1 }{ 2 } \cdot 8\cdot \frac { 16 }{ 3 } =\frac { 64 }{ 3 } .

As O is the centroid, [ B O C ] = 1 3 [ A B C ] [BOC] = \frac { 1 }{ 3 }[ABC] , because they have same base B C BC and it height is 1 3 \frac { 1 }{ 3 } of triangle A B C ABC height.

So, we get [ A B C ] = 3 64 3 = 64 [ABC]=3\cdot \frac { 64 }{ 3 } = 64 .

finaly, x = 64 x = 64 and ( x + 8 ) = 72 (x+8)=\boxed{72}

John Mistele
Apr 3, 2014

The medians are perpendicular, so consider EC as the base of triangles BEC and DEC. The sum of the areas of BEC and DEC is (BD)(EC)/2. Also, triangle AED is similar to triangle ABC with ratio 1:2, so the area of triangle AED is one fourth the area of ABC.

Our final equation is: (8*12/2)(4/3) = 72, which is the answer.

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