Тригонометрия - половинный угол
cos(α/2) = ±√((1 + cosα) / 2)
sin(α/2) = ±√((1 - cosα) / 2)
tg(α/2) = ±√((1 + cosα) / (1 - cosα))
tg(α/2) = sinα / (1 + cosα)
tg(α/2) = (1 - cosα) / sinα
cos(α/2) = ±√((1 + cosα) / 2) sin(α/2) = ±√((1 - cosα) / 2) tg(α/2) = ±√((1 + cosα) / (1 - cosα)) tg(α/2) = sinα / (1 + cosα) tg(α/2) = (1 - cosα) / sinα