Prerequisites:
Area of a right triangle (proof)
Proof:
In the given triangle ABC, draw an altitude from A to join BC at D. As a result, \triangle ABC is divided into two right triangles, i.e., \triangle ABD and \triangle ACD.
Since, these are right triangles, hence there area is given by:
\qquad\quad\text{Area of $\triangle ABD$} = \dfrac{1}{2} (AD).(BD)\\ \text{And}\quad\text{Area of $\triangle ACD$} = \dfrac{1}{2} (AD).(CD)
\begin{align}\therefore\;\;\quad\text{Area of $\triangle ABC$} &= \text{Area of $\triangle ABD$} + \text{Area of $\triangle ACD$}\\ &= \frac{1}{2} (AD).(BD) + \frac{1}{2} (AD).(CD)\\ &= \frac{1}{2} (AD).(BD + CD)\\ &= \frac{1}{2} (AD).(BC)\end{align}
Hence the result.
Recommended:
Area of rectangle
Area of rhombus
Isosceles triangle theorem
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