−−−> $\sqrt{sinA.sinC}=cos \frac{B}{2}$Do $\sqrt{sinA}, \sqrt{cos\frac{B}{2}}, \sqrt{sinC} $ lập thành CSN
=> $sinA.sinC=cos^2 \frac{B}{2}$
=> $-\frac{1}{2}(cos(A+C)-cos(A-C))=cos^2\frac{B}{2}$
=> $cos(A-C)-cos(180-B)=1+cosB$
=> $cos(A-C)+cosB=1+cosB$
=> $cos(A-C)=1$ => $A=C$
=> $\Delta ABC$ cân tại B