Sagot :
Salut,
1) tu as tan55° = HS/HC et tan75° = HS/HB donc tan75°/tan55° = HS/HB * HC/HS donc
tan75°/tan55° = HC/HB
2) on a tan75°/tan55° = HC/HB or HC = HB + BC donc
tan75°/tan55° = (HB + BC)/HB = HB/HB + BC/HB = 1 + BC/HB donc BC/HB = tan75°/tan55° - 1