Question 31180
Given ANY triangle ABC where 


length a is opposite angle A
length b is opposite angle B
length c is opposite angle C


then knowing just 3 sides, we can find an angle using the cosine rule, which states that:


{{{ a^2 = b^2 + c^2 -2bc*cosA }}}


so put the values in, re-arrange to find cosA and hence find the angle A.


Note: we could have the formula as
{{{ b^2 = a^2 + c^2 -2ac*cosB }}} or
{{{ c^2 = a^2 + b^2 -2ab*cosC }}}


it is completely flexible.


jon.