Prove that the line of centres of two intersecting circles subtends equal angles at the two points of intersection

Given: Two circles with centersAandB, which intersect each other atCandD.To prove :∠ACB=∠ADBConstruction : JoinAC,AD,BDandBC.Proof : In trianglesACBandADB, we haveAC=AD...Radii of the same circleBC=BD...Radii of the same circleAB=AB...CommonTherefore, by SSS criterion of congruence,△ACB≅△ADB⇒∠ACB=∠ADB...CPCT