∵D是线段BC的中点∴BC=2DC∵DC=3∴BC=6∵BC=3AB∴AB=2 ∴AC=AB+BC=2+6=82、∵AC=a∴AB=a/4 BC=3a/4∴BD=3a/8∴AD=AB+BC=a/4+3a/8=5a/8
