1. The problem statement, all variables and given/known data See attached picture. 2. Relevant equations 3. The attempt at a solution I have the solution, but I'm puzzled by the first step in their solution, which reads Xa^(2) + Xb^(2) = L^(2) The solution provider then takes the time derivative of this expression which then produces 2Xa(Xa(dot))+2Xb(Xb(dot)) = 0 where Xa(dot) and Xb(dot) is the Xa or Xb symbol with a dot over it. Thanks for you're assistance!