1. The problem statement, all variables and given/known data http://img267.imageshack.us/img267/8924/screenshot20120118at121.png [Broken] 3. The attempt at a solutionWe have that X = A + B. To show that X is unique, let two such sums be denoted by X1 X2 such that X1 ≠ X2. We write, X1 = A + B X2 = A + B The equations imply, X1 - A - B = 0 X2 - A - B = 0 Which imply, X1 - A - B = X2 - A - B. If we add vectors to both sides, X1 - A - B + A + B = X2 - A - B + A + B X1 + 0 + 0 = X2 + 0 + 0 X1 = X2, which contradicts our assertion that X1 ≠ X2. This shows that such an X is unique.