28.
29a. By crossing the X and Y axes at the zero point, the entire
coordinate system is divided into four equal parts, called
QUADRANTS.
These quadrants are numbered with Roman
numerals, beginning with the quadrant to the right of the Y axis
and above the +X axis.
The numerals increase in a
counterclockwise direction (see figure A, below left). Vectors
always start from the point of origin.
Their angle, used to
denote
vector
direction
of
rotation,
is
measured
COUNTERCLOCKWISE starting from the +X axis.
Angles
between 0 and 90 are in the first quadrant (I). Angles between
Figure A
90 and 180 fall into the second quadrant (II).
The third
quadrant (III) includes angles between 180 and 270.
The
fourth and final quadrant (IV) completes the circle and includes
angles between 270 and 360 (see figure B to the left).
Figure B
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IT0350
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