Solution 4.3:1a
From Förberedande kurs i matematik 1
If we draw the angle \displaystyle {\pi }/{5}\; on a unit circle, then it will have an \displaystyle x -coordinate that is equal to \displaystyle {\cos \pi }/{5}\; .
FIGURE 1 FIGURE 2 the line \displaystyle x={\cos \pi }/{5}\;x={\cos \pi }/{5}\;
In the figures, we see also that the only other angle between
\displaystyle 0
and
\displaystyle 2\pi
which has the same cosine value, i.e. same
\displaystyle x
-coordinate, is the angle
\displaystyle v=-\frac{\pi }{5}+2\pi =\frac{9\pi }{5}
on the opposite side of the
\displaystyle x
-axis.
