abaikan coretan biru

BC = √(AC² - AB²)
BC = √(20² - 12²)
BC = √(400 - 144)
BC = √256
BC = 16 cm

BD = √(AD² - AB²)
BD = √(13² - 12²)
BD = √(169 - 144)
BD = √25
BD = 5 cm

CD = BC - BD
CD = 16 - 5
CD = 11 cm

semoga membantu ^^