bonjour,
BC² = 400 ² + 300²
BC² = 250 000
BC = √250 000 = 500
BC = 500 m
AC/AD = AB/AE = CB/DE
300/AD = 400/1000 ⇔ AD = 300000 : 400 = 750
AD = 750 m
CD = 750 - 300 = 450
CD = 450 m
DE
400/1000 = 500/DE ⇔ 500 000 : 400 = 1 250
DE = 1 250 m
400 + 500 + 450 + 1 250 = 2 600 m
