Cosine theorem: for any triangle, the square of any side is equal to the sum of the squares of the other two sides MINUS twice the cosine of the angle between these two sides and them. If a triangle with three side a, b and c is a, b and c respectively, Then the property-A2 = B2+C2-2bcCosab2 = A2+C2-2acCosbC2 = A2+B2-2abCoscCosc = Cosb = (A2+C2-B2)/(2ac) Cosa = (C2+B2-A2)/(2bc) is satisfied (also used in physics. Triangle area: both sides of the triangle, then s = 1/2 ABS Inc or: given three sides of a triangle, then (Helen formula) s = √ [p (p-a) (p-b) (p-c)] (p = (a+b+c)/2.