This wiki is about problem solving on triangles. You need to be familiar with some (if not all) theorems on triangles.
Triangles and are equilateral triangles of the same size. If and , find the area of triangle .
Since is isosceles,
It then follows that
By the exterior angle theorem,
By applying sine law on , we have
Note that the area of a triangle is half the product of two adjacent sides multiplied by the sine of the included angle. We have