1 cos(b/2)=1/{1+[tan(b/2)]^2}=2/根号5
sin(b/2)=cos(b/2)*tan(b/2)=1/根号5
sinb=2*cos(b/2)*sin(b/2)=4/5
cosb=2*[cos(b/2)]^2-1=3/5
2 因为a+b>a 又 sin(a+b)
1 cos(b/2)=1/{1+[tan(b/2)]^2}=2/根号5
sin(b/2)=cos(b/2)*tan(b/2)=1/根号5
sinb=2*cos(b/2)*sin(b/2)=4/5
cosb=2*[cos(b/2)]^2-1=3/5
2 因为a+b>a 又 sin(a+b)