(tgx+tgy)/(ctgx+ctgy)=ctgx*ctgy
1. tgx+tgy=sinx/cosx+siny/cosy=(sinx*cosy+siny*cox)/(cox*cosy)=sin(x+y)/(cosx*cosy)
2. ctgx+ctgy=cosx/sinx+cosy/siny=(cosx*siny+cosy*sinx)/(sinx*siny)=sin(x+y)/(sinx*siny)
3. [sin(x+y)/(cosx*cosy)] / [sin(x+y)/(sinx*siny)]=(cosx*cosy)/(sinx*siny)=(cosx/sinx)*(cosy/siny)=ctgsx*ctgy