1. The problem statement, all variables and given/known data Prove: [tex]\cos\alpha\cdot\cos\alpha'+\cos\beta\cdot\cos\beta'+\cos\gamma\cdot\cos \gamma'=\cos\theta[/tex] See drawing Snap1 2. Relevant equations None 3. The attempt at a solution See drawing Snap2. i make the length of the lines 1 and 2 to equal one, for simplicity. The projection of line 1 on one of the axes is cos(α). ##\cos\alpha\cdot\cos\beta## is the projection of line OA=cos(α) on line 2, which causes line AB to be perpendicular to line 2. If i will make the same procedure for all 3 axes and add the 3 projections on line 2 i have to get, see Snap1, line OC which is ##1\cdot\cos\theta##, but i don't know how to do it. The book from which i took this problem says i have to solve it from geometrical considerations. Where can i find the proof to this problem?