given:- AC=AD
BC=AC
In △CAB △DAB
AB=AB (Common)
AC=BD (given )
BC=AD (given)
∴ lySSS
△CAB≅△DAB
ly CPCT→
∠ACB=∠ADB−−−−(1)
In △AKC & △ABKC
AC=BDK(ly−−−(1))
∠AKC=∠BKD (vertically opposite angle)
∴ lyAAS−
△ AKC≅△BKD
ly CPCT⇒
AK+BK
BC=AD
BC−AK=AD−AK
BC−BK=AD−AK (AK−BK)
CK−DK