cos45*sin225+0.5ctg(-315)-cos270

сos45°·sin225°+0.5ctg(-315°)-cos270°=сos45°·sin(270°-45°)-0,5ctg315°-cos270°=cos45°·(-cos45°)-0,5ctg(360°-45°)=-cos²45°+0,5ctg45°+0=
=-(√2/2)²+0,5·1=-2/4+0,5=-0,5+0,5=0