申論題

2.The points A(1,-21), B(0,1,6) and C(-3,4,-2) form the vertices of a triangle. Calculate the angle between the line AB and the line from A to the midpoint P of . You do not need to give the explicit value of the angle. Just express the angle using inverse trigonometric functions. Hint: use dot product for two vectors AB and AP which is a scalar equal to the multiplication of the lengths of the two vectors and the cosine of the angle between the two vectors.