∵∠CAD = ∠B ∠c为公共角∴△CAD ∽ △CBA 即：AC:BC = AD:AB = 6:8 ① AC = 3/4BC AC:CD = AD:AB = 6:8 ② AC = 4/3CD 3/4(7 + CD) = 4/3CD 化简 CD = 9
