1. The problem statement, all variables and given/known data Prove Lagrange's identity for real numbers http://mathworld.wolfram.com/LagrangesIdentity.html 3. The attempt at a solution I tried one of the methods used in proving the Cauchy-Schwarz inequality (Ax^2 + Bx + C is greater than or equal to zero, where a = the sum from k=1 to n of (ak)^2, b = the sum from k=1 to n of (ak*bk), and c = the sum from k=1 to n of (bk)^2), but I couldn't get very far because I don't understand the last term of the equation's right side. Anyone have any ideas on how to start the problem? Thanks.