If we study the positive Xuan theorem and cosine theorem, then: area S=absinx/2. X is the angle corresponding to the C side. Cosx = (a 2+b 2-c 2)/(2ab), then sinx= under the radical sign (1-the square of cosx), and the final area is: let p=(a+b+c)/2.

The square of the area s =p(p-a)(p-b)(p-c) is obtained from the average inequality: S2 = p (p-a) (p-b) (p-c) "p * (p-a)+(p-b)+(p).