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BC = 20 cm
BD = 16 cm
CD = 20 cm - 16 cm = 4 cm

AD = √BD x CD
      = √16 cm x 4 cm
      = √64
      = 8 cm

ad= √bd. cd
= √16. 4
= √ 64
= 8

JADIKAN JAWABAN TERBAIK YA