According to Pythagorean theorem, we get: ∫∠ADC = 90, ∠ ADC+∠ ADB = 180.

∴AC？ =AD？ +DC？ =225= 15? ∴∠ADB=90

AC > 0 means ∠ ADC = 90, ∠ ADB = 90, ∠ bad = 90.

∴AC= 15

∴∠ADC=90

AB = 20，AC= 15，BC=25

∴BC？ =AB？ +AC？ =625=25?