Refer to 7(b):
Help, please? The answer given in the textbook states 57.5 degrees, but I can't seem to get it.
A slightly different method: Set up a coordinate system with origin at A, positive x-axis from A to B, positive y-axis from A to D, and positive z-axis upward. Then the vector AB is <12, 0, 0> and AP is <6, 8, 5>. The magnitude of AB is 12, the magnitude of AP is , and the dot product of A and B is 6(12)= 72. We must have so that .