sinB=2sinA*cosC
sin(A+C)=2sinAcosC
sinAcosC+cosAsinC=2sinAcosC
sinAcosC-cosAsinC=0
sin(A-C)=0
A=C,B=2A最大角B
sin(B/2)=3/4,cos(B/2)=√7/4
sinB=2sin(B/2)cos(B/2)=3√7/8
B=180°-arcsin(3√7/8)