sinB*sinC=(cosA/2)^2
sinB*sinC=(cosA+1)/2
2sinBsinC=cosA+1
2sinBsinC=1-cos(B+C)
2sinBsinC=1-cosBcosC+sinBsinC
cosBcosC+sinBsinC=1
cos(B-C)=1
B-C=0
等腰三角形
sinB*sinC=(cosA/2)^2
sinB*sinC=(cosA+1)/2
2sinBsinC=cosA+1
2sinBsinC=1-cos(B+C)
2sinBsinC=1-cosBcosC+sinBsinC
cosBcosC+sinBsinC=1
cos(B-C)=1
B-C=0
等腰三角形