The representative of my math class explained. Because two straight lines intersect, a complementary angle will be formed. Then mathematicians stipulate that the smaller angle is the included angle between two straight lines. That's why you need to write the angle or its complement.

(The following is my personal understanding. ) and the range of values of line angle and plane angle is [0, π/2], according to the way they find the angle (that is, making a vertical line or something, you should know, right? ), there is no need to write or its complement.

So the key should be to understand the definition of line angle, line angle and surface angle.