Question 1058523
{{{drawing(400,400,-3.5,3.5,-3.5,3.5,
graph(400,400,-3.5,3.5,-3.5,3.5),
locate(-2.8,.3,"(-2,0)"),circle(-2,0,.05),
green(line(0,0,-2,0)), red(arc(0,0,1,-1,0,180)) )}}}<pre>
The angle indicated by the red arc is 180° or <font face="symbol">p</font> radians.

{{{matrix(1,5,

cosine,
""="",
adjacent/hypotenuse,
""="",
x/r


)}}}

But the right triangle is "smashed" flat against the x-axis,
so its hypotenuse coincides with its horizontal side, and
its vertical side is 0. 

The adjacent side is the x coordinate of the point -2.
The hypotenuse r, is the radius vector which coincides 
with the green line.  Since hypotenuse or radius vector
is always taken positive, its value is +2

{{{cos("180°")}}}{{{""=""}}}{{{x/r}}}{{{""=""}}}{{{(-2)/("" + 2)}}}{{{""=""}}}{{{-1}}}

Edwin</pre>