GiventhatinthequadrilateralABCDAD=DC=AC=a=20units.∴ΔACDisanequilateraltriangle.Soitsarea=√34a2=√34202unit2=173.2unit2.AgaininΔABCthe∠B=90o&AB⊥AC.SoΔABCisarightonewithhypotenuseAC=20units,BC=12units&AB=xunits.ApplyingPythagorastheorem,weget,AB=x=√AC2−BC2=√202−122units=16units.∴arΔABC=12×AB×BC=12×16×12unit2=96unit2.Soar.quadrilateralABCD=arΔACD+arΔABC=(173.2+96)unit2=269.2unit2.Ans−arquadrilateralABCD=269.2unit2.