,线段AB上有两点C、D,AC:BC=5:7,AD：BD=5:11,且CD=15cm,求AB的长.

AC:BC=5:7
AC:(AC+BC)=5:(5+7)
AC:AB=5:12
AC=5AB/12
AD:BD=5:11
AD:(AD+BD)=5:(5+11)
AD:AB=5:16
AD=5AB/16
CD=AC-AD=5AB/12-5AB/16
=5AB(4-3)/48
=5AB/48=15
AB/48=3
AB=48*3=144cm