ABCD
P = 2 × ( L + l) = 18 cm
L + l = 18 : 2 = 9 cm
L = AB
l = BC
L = 2 × l
2 × l + l = 9
3 × l = 9
l = 9 : 3
l = BC = 3 cm
L = AB = 2 × 3 = 6 cm
D C
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A B