## Elementary Geometry for College Students (5th Edition)

$f=10sin(36)$
Since this is a pentagon, the angle is given by: 360/10=36 If we draw a triangle extending to the center point between each corner, we find that the length of this side $f/2$ is: $sin(36)= (f/2)/a$ This gives: $2asin(36)=f$ Since a=5: $f=10sin(36)$