asinA=bsinB=2R⇒{sinA=a2RsinB=b2RasinA=bsinB=2R⇒{sinA=a2RsinB=b2R
c2=a2+b2−2bacosC⇒cosC=a2+b2−c22abc2=a2+b2−2bacosC⇒cosC=a2+b2−c22ab
dt⇔a2R=2.b2R.a2+b2−c22abdt⇔a2R=2.b2R.a2+b2−c22ab
⇔a=a2+b2−c2a⇔a2=a2+b2−c2⇔a=a2+b2−c2a⇔a2=a2+b2−c2
⇒b2=c2⇒b=c⇒b2=c2⇒b=c
Vậy tam giác ABC cân tại A