tg(α−β) = (tgα - tgβ) / (1 + tgαtgβ)4 = (3 - tgβ) / (1 + 3tgβ)4 + 12tgβ = 3 - tgβ13tgβ = -1tgβ = -1/13
tg(α−β) = (tgα - tgβ) / (1 + tgαtgβ)
4 = (3 - tgβ) / (1 + 3tgβ)
4 + 12tgβ = 3 - tgβ
13tgβ = -1
tgβ = -1/13