$解：?(1)?∵?AC?平分?∠BAD?$
$∴?∠BAC=∠CAD?$
$∵?BC⊥AC，??CD⊥AD?$
$∴?∠ACB=∠ADC=90°?$
$∴?△ABC∽△ACD?$
$∴?\frac {AB}{AC}=\frac {AC}{AD}?$
$∵?AB=18，??AC=12?$
$∴?\frac {18}{12}=\frac {12}{AD}?$
$∴?AD=8?$
$?(2)?∵?△ABC∽△ACD?$
$∴?\frac {DE}{CF}=\frac {AC}{AB}=\frac 23?$
