a)
AD = √(AB² - BD²) = √(100 - 36) = √64 = 8 cm
AD² = BD·DC = > DC = AD²/BD = 64/4 = 16 cm
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b)
AD = √(AC² - DC) = √(144 - 36) = √108 = 6√3 cm
AD² = BD·DC = > BD = AD²/DC = 108/6 = 18 cm