sina=2tan(a/2)/[1+tan²(a/2)]=2×(1/3)/[1+1/9]=3/5tan(a/2)=1/3sin(a/2)/cos(a/2)>0sin(a/2)cos(a/2)>0那么sin(a/2)和cos(a/2)同号在直角三角形里求cos(a/2)此时a/2对的直角边是1,临边是3,斜边是√10cos(a/2)...