13.易知AB=10,AC=5√3.
(1)作OD⊥AC于D,由△AOD∽△ABC得OD/BC=AO/AB=AD/AC,
∴OD=x/2,AD=x√3/2,
当x/2>1,即x>2时AC与圆O相离;x=2时AC与圆O相切;x