cos^2A+cos^2B>1+cos^2C 1-sin^2A+1-sin^2B>1+1-sin^2C sin^2C>sin^2A+sin^2B a/sinA=b/sinB=c/sinC c^2>a^2+b^2 cosC=(a^2+b^2-c^2)/2ab＜0,所以c为钝角 三角形ABC是钝角三角形.
