Keliling bangun ACDE adalah

∆ABE kongruen dengan ∆BCD
AE = DC = 20 cm
BC = BE = 12 cm
AB = BD = √(20^2 - 12^2) = √(400 - 144) = √256 = 16 cm

DE = BD - BE = 16 - 12 = 4

Keliling ACDE
= AB + BC + CD + DE + AE
= 16 + 12 + 20 + 4 + 20
= 72 cm