A straight line has a negative gradient. What angle does it make with the positive horizontal direction?

1. The angle of inclination is measured from the positive horizontal direction up to the line. 2. A line with a positive gradient rises to the right, giving an acute angle. 3. A line with a negative gradient falls to the right instead. 4. The angle formed between such a line and the positive horizontal direction is obtuse. 5. A calculator returns a negative angle for a negative gradient, which lies in the fourth quadrant. 6. That negative angle is turned into the obtuse angle by adding half a revolution. 7. A zero angle belongs to a horizontal line, whose gradient is zero rather than negative. _Source: Siyavula Mathematics Grade 11 (Everything Maths, CC BY 4.0), Chapter 4: Analytical geometry_