Question 793768
Vertex of the equal sides has an altitude extending from it to the middle of the third side, intersecting this third side in the middle, forming two right triangles.  Each such triangle has hypotenuse 8 cm and short leg 2.5 cm.  


{{{sine(atVertex)=2.5/8}}} for each right triangle, but for the entire isosceles triangle, {{{sin(2*atVertex)=2.5/8}}}, so wanted is {{{2*arcsin(2.5/8)}}}.