Given In a circle AYDZBWCX, two chords AB and CD intersect at right angles. To prove arc CXA + arc DZB = arc AYD + arc BWC = Semi-circle. Construction Draw a diameter EF parallel to CD having centre M. Proof Since, CD||EF arc EC = arc PD … (i) arc ECXA = arc EWB [symmetrical about diameter of a circle] arc AF = arc BF …(ii)