Cosine Rule , what good is it ???? Does the Cosine rule hold true for say negative lengths ??? as in a vector quantity like displacement ??? I came across this problem which had -15km and 10km as the known sides whereas the angle opposite the unkown side is 60 degree .... I tries using c^2 = a^2 + b^2 - 2*a*b cosC but this didnt work .... then on account of frustration i forcibly changed the '-' part of the formula to a '+' only to find that it works ... ??? Can someone explain this to me ??