Question
In the adjoining figure, ABC and ABD are two triangles on the same base AB. If line segment CD is
bisected by AB at O, show that $\text{ar}(\triangle\text{ABC})=\text{ar}(\triangle\text{ABD}).$

Answer

We know that median of a triangle divides it into two triangles of equal area. Now, AO is the median of $\triangle\text{ACD}.$$\Rightarrow\ \text{A}(\triangle\text{COA})=\text{A}(\triangle\text{DOA})\dots(1)$
And, Bo is the median of $\triangle\text{BCD}.$$\Rightarrow\ \text{A}(\triangle\text{COB})=\text{A}(\triangle\text{DOB})\dots(2)$
Adding (1) and (2), we get:$\text{A}(\triangle\text{COA})+\text{A}(\triangle\text{COB})=\text{A}(\triangle\text{DOA})+\text{A}(\triangle\text{DOB})$
$\Rightarrow\ \text{A}(\triangle\text{ABC})=\text{A}(\triangle\text{ABD})$

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