Draw the altitude h perpendicular to the base which bisects 
the vertex angle 108° into two 54° angles and bisects the base 
into two parts each 10 cm in length.  It also forms two congruent
right triangles.



Looking at the right triangle on the left, the side opposite
the 54° angle is 10 and the side adjacent to the 54° angle
is the unknown side h.  We know that the tangent is the
opposite over the adjacent, so we have this equation:



We put 1 under the tan(54°)



We can cross multiply and get



Then we divide both sides by tan(54°)



Now get your calculator and make sure
it is in degree mode.



Edwin