help with geometry

What do you know about cyclical quadrilaterals in a circle ?  ( opposite angles sum to 180^o )

    What do you know about interna angles in a pentagon?  (108^o )

Now look at this diagram:

an angle in a regular pentagon is 108^o and the triangle is isosceles so the measure of DEB is 108-((180-108)/2)=108-(72/2)=108-36=72^o sp we have that \(\angle DEB\) is 72^o or \(\boxed{72^\circ}\).