1)cos120=cos(90+30)=sin30=1/2
2)tg5π/4=tg5*180/4=tg225=tg(180+45)=1
3)sin210=sin(180+30)=-1/2
4)ctg7π/4=ctg315=ctg(270+45)=-tg45=-1
1)sin(a-b)+sinbcosa=sinacosb-cosasinb+sinbcosa=sinacosb=3/5*(-7/15)=-7/25
2)cos(a+b)+sinasinb=cosacosb-sinasinb+sinasinb=cosacosb=(-2/5)*15/16=-3/8