Ctg1200,tg750,tg(-870),cos330,cos(-1230)

Ctg1200=ctg(1080+120)=ctg120=ctg(90+30)=-tg30=-1/√3
tg750=tg(720+30)=tg30=1/√3
tg(-870)=-tg870=-tg(720+150)=-tg150=-tg(180-30)=tg30=1/√3
cos330=cos(360-30)=cos30=√3/2
cos(-1230)=cos1230=cos(1440-210)=cos210=cos(180+30)=-cos30=-√3/2