1. The problem statement, all variables and given/known data Prove the law of cosines: If v and w are vectors in R^2, then |v-w|^2 = |v|^2 + |w|^2 - 2|v||w|cosθ where θ is the angle between v and w. 2. Relevant equations/info |x| refers to length, and not absolute value (If that doesn't make any sense then I must be wrong, but our prof. insisted on only using single lines to define a value as 'length', which can be confusing at times). 3. The attempt at a solution I got blindsided by this question (missed the lecture..). Even just a starting point, or even an online resource would be greatly appreciated, and to go as far fully providing me with the solution would be extremely helpful (but I suppose possibly detrimental). Also, the rest of the assignment deals with orthogonal projections, and cross products, as well as distances from points to planes, and lines - do these sort of questions involve the usage of the above law?