tanA/tanB=(2sinC-sinB)/sinB
(sinAcosB)/(cosAsinB)=(2sinC-sinB)/sinB
sinAcosB=2sinCcosA-sinBcosA
sin(A+B)=2sinCcosA
cosA=1/2
A=60°
tanA/tanB=(2sinC-sinB)/sinB
(sinAcosB)/(cosAsinB)=(2sinC-sinB)/sinB
sinAcosB=2sinCcosA-sinBcosA
sin(A+B)=2sinCcosA
cosA=1/2
A=60°