Вычеслить sin690°ctg1200°-1/2tg750°

690°=360*2-30°   1200°=360°*3+120°  750=360°*2+30°
sin690°ctg1200°-1/2tg750°=sin(-30°)ctg(90°+30)-tg30°/2=
=-1/2*(-√3/3)-√3/6=√3/6-√3/6=0
учтено ctg(90°+30)=-tg30

Sin690*ctg1200-1/2*tg750=sin(2*360-30)*ctg(180*7-60)-1/2tg(180*4+30)=
=-sin30*(-ctg60)-1/2*tg30=-1/2*(-√3/3)-1/2*√3/3=√3/6-√3/6=0