$2cotA=cotB+cotC\Leftrightarrow 2\frac{cosA}{sinA}=\frac{cosB}{sinB}+\frac{cosC}{sinC}\Leftrightarrow 2\frac{cosA}{a}=\frac{cosB}{b}+\frac{cosC}{c}\Leftrightarrow 2(b^{2}+c^{2}-a^{2})=2a^{2}\Leftrightarrow 2a^{2}=b^{2}+c^{2}$(Đúng theo gt).
$2cotA=cotB+cotC\Leftrightarrow 2\frac{cosA}{sinA}=\frac{cosB}{sinB}+\frac{cosC}{sinC}\Leftrightarrow 2\frac{cosA}{a}+\frac{cosB}{b}+\frac{cosC}{c}\Leftrightarrow 2(b^{2}+c^{2}-a^{2})=2a^{2}\Leftrightarrow 2a^{2}=b^{2}+c^{2}$(Đúng theo gt).
$2cotA=cotB+cotC\Leftrightarrow 2\frac{cosA}{sinA}=\frac{cosB}{sinB}+\frac{cosC}{sinC}\Leftrightarrow 2\frac{cosA}{a}
=\frac{cosB}{b}+\frac{cosC}{c}\Leftrightarrow 2(b^{2}+c^{2}-a^{2})=2a^{2}\Leftrightarrow 2a^{2}=b^{2}+c^{2}$(Đúng theo gt).