Line through origin, with slope passes through 2nd and 4th quadrants.
The angle is the acute angle the line makes with the negative x-axis in the 2nd quadrant
or with the positive x-axis in the 4th quadrant.
The angle 4t is the pair of counterclockwise angles the line with negative slope
makes with the positive x-axis.
In the 2nd quadrant...
For other solutions, add integer multiples of
In the attachment,