<<Mentor note: Missing template due to originally being posted elsewhere>> Hello everyone. I have the following problem: Determine the angles of a triangle where two sides of a triangle are formed by the vectors A = 3i -4j -k and B=4i -j + 3k I thought that I would find the third side being represented by vector R which would be equal to A-B, that is R = -i -3j -4k. I would then take the cross products of each combination of the two vectors and find the angle between them and these would be the angles of the triangle. Dot producting A and B vectors, I get the angle between them to be 60 degrees. Dot producting the A and R vectors, I also get 60 degrees. Thus with the final angle = 180 - 60 - 60, the last angle should be 60 degrees also. This is blatantly wrong as my solutions tell me that the answer is: arcos 7 /sqrt(75), arcos sqrt(26)/sqrt(75), 90 degrees, or 36degrees4', 53degrees56', 90 degrees Could anyone please direct me as to what I did wrong and what mistakes need to be fixed? Thanks in advance.