Case 1.
sinangleCAD=SinA=(CD)/(AC)sin∠CAD=sinA=CDAC
=> CD=AC*sinA=b*sinA⇒CD=AC⋅sinA=b⋅sinA --- (1)
sinangleCBD=SinB=(CD)/(BC)sin∠CBD=sinB=CDBC
=> CD=BC*SinB=a*sinB⇒CD=BC⋅sinB=a⋅sinB ---(2)
(1) = (2), => b*SinA=a*SinB⇒b⋅sinA=a⋅sinB
=> SinA/a=sinB/b⇒sinAa=sinBb
Case 2.
sinangleCAD=SinA=(CD)/(AC)sin∠CAD=sinA=CDAC
=> CD=AC*sinA=b*sinA⇒CD=AC⋅sinA=b⋅sinA --- (1)
angleCBA=angleB∠CBA=∠B
angleCBD=180-angleB∠CBD=180−∠B
sinangleCBD=Sin(180-B)=sinB=(CD)/(BC)sin∠CBD=sin(180−B)=sinB=CDBC,
=> CD=BC*SinB=a*sinB⇒CD=BC⋅sinB=a⋅sinB ------(2)
(1) = (2), => b*SinA=a*SinB⇒b⋅sinA=a⋅sinB
=> SinA/a=sinB/b⇒sinAa=sinBb