фи или пси будет b
tg(a) + tg(-b) = tg(a) - tg(b) = sin(a)/cos(a) - sin(b)/cos(b) = (sin(a)cos(b) - sin(b)cos(a))/cos(a)cos(b) = sin(a - b)/cos(a)cos(b)
ctg(-a) + ctg(b) = - ctg(a) + ctg(b) = cos(b)/sin(b) - cos(a)/sin(a) = (sin(a)cos(b) - sin(b)cos(a))/sin(a)sin(b) = sin(a - b)/sin(a)sin(b)
sin(a - b)/cos(a)cos(b) : sin(a - b)/sin(a)sin(b) = sin(a - b)/cos(a)cos(b) * sin(a)sin(b)/sin(a - b) = sin(a)sin(b) / cos(a)cos(b) = tg(a)tg(b)
