Salut! =)
Semiperimetru : ( AB + AC + BC ) / 2
( AB + AC + BC ) / 2 = 18 => AB + AC + BC = 36
AB = 2AC
BC = AC - 20
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AB , AC , BC = ?
Inlocuim in formula semiperimetrului =>
2AC + AC + AC - 20 = 36
2AC + AC + AC = 56
4AC = 56
AC = 14 cm
AB = 2 × 14 => AB = 28 cm
BC = AC - 20 = 14 - 20 = -6 cm
Bafta! =)