円の半径をr=36とすると、四角形ADECは台形になり、CE=r*sin(20), AD=r*sin(70)、DE=r*cos(20)-r*cos(70) より
S=(CE+AD)*DE/2=r{sin(20)+sin(70)}*r{cos(20)-cos(70)}/2=(r^2/2)*{sin(20)+sin(70)}{cos(20)-cos(70)}
=(r^2/2)*sin(50)=648*sin(50)=496.396.....