1. The problem statement, all variables and given/known data Prove that d/dt[r.(vxa)] = r.(vxda/dt) 2. Relevant equations r, v, a are position, velocity and acceleration vectors. ..r.(v.. is the dot product. ..vxa.. is the cross product 3. The attempt at a solution I expand the equation using the product rule for dot and cross products to get: dr/dt.(vxa)+r.(dv/dt x a+v x da/dt) I've expanded this further on paper using the x,y,z components of each vector but i can't manipulate it to get the desired result? Have I missed a step or overlooked something here?