sinBsinC=(cosA/2)^22sinBsinC=2(cosA/2)^22sinBsinC=1+cosA2sinBsinC=1-cos(B+C)2sinBsinC=1-cosBcosC+sinBsinCcosBcosC+sinBsinC=1cos(B-C)=1B-C=0B=C所以,三角形ABC是等腰三角形AAAAAAAAAAA
