COS(A-30度)+SINA=4/5 乘根号3 求SIN(A+210度)?

根号3用[3]代替
COS(A-30)+SINA=COSA*[3]/2+SINA*1/2+SINA=4*[3]/5
两边同时约去[3]
COSA*1/2+SINA*[3]/2=4/5
COS(A-60)=4/5
COS(A-60+180)=-4/5
COS(A+120)=-4/5
COS(A+120)=SIN(A+120+90)=-4/5
SIN(A+210)=-4/5