Area of Isosceles Triangle/Proof 2
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Theorem
Let $\triangle ABC$ be an isosceles triangle whose apex is $A$.
Let $\theta$ be the angle of the apex $A$.
Let $r$ be the length of a leg of $\triangle ABC$.
Then the area $\AA$ of $\triangle ABC$ is given by:
- $\AA = \dfrac 1 2 r^2 \sin \theta$
Proof
A direct application of Area of Triangle in Terms of Two Sides and Angle:
- $\AA = \dfrac 1 2 a b \sin \theta$
where $a = b = r$.
$\blacksquare$