Question

Can someone help me find what the highlighted part came from?

Answer 1

Answer:

$\cos(\frac25\pi) = \frac14(\sqrt5-1) \approx 0.3090$

$\sin(\frac25\pi) = \sqrt{\frac58 + \frac{\sqrt5}{8}} \approx 0.9511$

$\cos(2\pi) = 1$

The value of $\frac{2\pi }{5}$ radians corresponds to an angle of 180/5 = 72 degrees.