cos 23pi/4=cos 7pi/4=cos(2pi-pi/4)=cos (-pi/4)=sqrt2/2
sin 15pi/4=sin 7pi/4=sin(2pi-pi/4)=sin (-pi/4)=-sin(pi/4)=-sqrt2/2
ctg(-11pi/2)=-ctg(pi/2)=0
cos 23pi/4-sin 15pi/4=sqrt2/2+sqrt2/2=sqrt2
cos 23pi/4=cos 7pi/4=cos(2pi-pi/4)=cos (-pi/4)=sqrt2/2
sin 15pi/4=sin 7pi/4=sin(2pi-pi/4)=sin (-pi/4)=-sin(pi/4)=-sqrt2/2
ctg(-11pi/2)=-ctg(pi/2)=0
cos 23pi/4-sin 15pi/4=sqrt2/2+sqrt2/2=sqrt2