(x)/(x-3) - (5)/(x+3) = (18)/(x^(2)-9)
sin(π+a) = -sina
cos(π/2+a) = -sina
5*(-0.5) + (-0.5)
-2.5 - 0.5 = -3
tg(α-пи/2)=-tg(пи/2-α)=-ctgα