1. The problem statement, all variables and given/known data Find the angle between a) The line L1 given by the equations y = 2z, x = 0 and b) The line L2 given by the equations x = 3z, y = 0. 2. Relevant equations v.u=|v|*|u|*cos(θ) 3. The attempt at a solution I know that I need to basically have a vector for each line to substitute into the equation above, but I am confused as to which set of the following vectors is correct: 1) L1 can be represented as 0x+1y-2z=0 so the vector/norm of it would be: (0,1,-2) (the tutor in class used norms when someone asked her about the question but I had trouble following what she was doing. :S) L2 would similarly be (1,0,-3). This produces an angle of 31.95° 2) Let z=1 in both cases, such that: L1 = (0,2,1) and L2 = (3,0,1). This when entered into the formula generates an angle of 81.87°. I can see the justification behind both answers but I don't know which would be correct? Please help! Thank you!