tg(a+b) = (tga + tgb) / (1 - tga tgb)
tg(a+b) = (5/3 + 4/3) / (1 - 5/3 4/3)
tg(a+b) = 9/3 / (1 - 20/9)
tg(a+b) = 3 / (1 - 20/9)
tg(a+b) = 3 / (9/9 - 20/9)
tg(a+b) = 3 / (-11/9)
tg(a+b) = -27/11