【】在三角形内？sin(A+B)/2=sinC/2sin(A+B)/2*cos(A+B)/2=sinC/2*cos(A+B)/21/2*sin(A+B)=sinC/2*cos(A+B)/21/2*sinC=sinC/2*cos(A+B)/2sinC/2*cosC/2=sinC/2*cos(A+B)/2cosC/2=cos(A+B)/2=sinC/2tanC=1， C=45°A+B=90°等腰直角三角形
