I always seem to find this place whenever I'm in need of homework help, so I finally decided to register (and hopefully post in the right area). My given problem is Here are three displacements, each in meters: d1 = 4.2i + 2.7j - 7.9k, d2 = -1.0i + 2.0j + 3.0k, and d3 = 4.0i + 3.0j + 2.0k. What is r = d1 - d2 + d3 ((a), (b) and (c) for i, j and k components respectively)? (d) What is the angle between r and the positive z axis? (e) What is the component of d1 along the direction of d2? (f) What is the component of d1 that is perpendicular to the direction of d2 and in the plane of d1 and d2? The solutions I've gotten are a) 9.2 m b) 3.7 m c) -8.9 m d) 131.91° e) -6.01 m f) 7.16 m All of which I know to be correct. The confusion I'm having is with part f. I used this equation to solve for f: [(magnitude of d1)^2 - (the component of d1 along the direction of d2)^2]^1/2 I just don't understand why this works. Anyone help?