All the interior angles are of measure 60 degrees.
We want to find the height of this triangle. Call it h. On the left side of this illustration is a right triangle with height h and base (1/2)(10), or 5.
The ratio of height to base is the tangent of 60 degrees:
h
tan 60 degrees = --------------
5
h
and tan 60 degrees = 1.73 = ------
5
Multiplying both sides of this equation by 5 yields h = 8.66. This is |BD|. This is the same as Answer B.
