An introduction to mathematics, by A. N. Whitehead.

TRIGONOMETRY 185
YOY' perpendicular to it. Let any point P
at a distance r from O have coordinates x
and y. These coordinates are both positive
in the first "quadrant" of the plan, e.g. the
coordinates x and y of P in fig. 27. In the
A
P(^)' / / \ IA X
Y'
Fig. 27.
other quadrants, either one or both of the
coordinates are negative, for example, x' and
y for P', and x' and y' for P', and x' and y'
for P", and x and y' for P" in fig. 27, where
x' and y' are both negative numbers. The
positive angle POA is the arc AP divided
by r, its sine is Y and its cosine is -; the posir T