解:如图,作AF⊥BD,CE⊥BD,AG∥EF,AG=EF连接CG,则∠CAG就是异面直线BD与AC所成角,由题意,BD= 5 ,AF=CE= 2 5 5 ,DF=BE= 5 5 ,EF= 3 5 5 因为,二面角C-BD-A为直二面角,所以,△CEG和△AGC都是直角三角形,CG= 2 10 5 ,AC= ( 3 5 5 )2+(2