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Medians AD and BE of triangle ABC are perpendicular. If AD = 15 and BE = 20, then what is the area of triangle ABC?

 

- Daisy

 Aug 20, 2018
edited by dierdurst  Aug 22, 2018
edited by dierdurst  Aug 24, 2018
 #1
avatar+21848 
+5

Medians AD and BE of triangle ABC are perpendicular. If AD = 15 and BE = 20,

then what is the area of triangle ABC?

 

not true to scale

 

\(\begin{array}{|rcll|} \hline A_{\triangle ABC}=A &=& \dfrac{ab\sin{C}}{2} \\ \hline \\ A_{\triangle EDC} &=& \dfrac{\dfrac{ab\sin{C}}{4}}{2} \\\\ &=& \dfrac{\dfrac{ab\sin{C}}{2}}{4} \\\\ &=& \dfrac{A}{4} \\ \hline \\ A_{ABDE} &=& \dfrac{AD\cdot BE}{2} \\\\ &=& \dfrac{15\cdot 20}{2} \\\\ &=& 15\cdot 10 \\\\ &=& 150 \\ \hline \\ A &=& A_{\triangle EDC} + A_{ABDE} \\ A &=& \dfrac{A}{4} + 150 \quad & | \quad \cdot 4 \\ 4A &=& A + 600 \\ 3A &=& 600 \quad & | \quad :3 \\ \mathbf{A} &\mathbf{=}& \mathbf{200} \\ \hline \end{array}\)

 

The area of triangle ABC is 200

 

laugh

 Aug 20, 2018
edited by heureka  Aug 20, 2018
 #3
avatar
+1

CPPhill: I think heureka's reasoning comes from this property of irregular quadrilateral:

"The line joining the mid-points of any two adjacent sides is parallel to the corresponding diagonal and equal to half of it."

 Aug 20, 2018

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