Xét PT⇔sinAsinB=2cos(B+C2)cos(B−C2)2cos(A+C2)cos(A−C2)=sinA2.cosB−C2sinB2.cosA−C2⇔cosA2cosB2=cosB−C2cosA−C2
⇔cos(2A−C2)=coos(2B−C2)
⇒ˆA=ˆB⇒△ABC cân
Hoặc \widehat{A}+\widehat{B}=\widehat{C} \Rightarrow \widehat{C}=90^0 \Rightarrow \triangle ABC vuông