2cotA=cotB+cotC,
2cosA/sinA=cosB/sinB+cosC/sinC,
代入cosA=(b^2+c^2-a^2)/(2bc),sinA=a/2R等,约去2R,得
2(b^2+c^2-a^2)/(2abc)=(c^2+a^2-b^2)/(2abc)+(a^2+b^2-c^2)/(2abc0,
2a^2=b^2+c^2.