Solution :
The value of cos 72 degrees is \(\sqrt{5} – 1\over 4\).
Proof :
We know that value of sin 18 degrees is \(\sqrt{5} – 1\over 4\).
Since 72 degree is the complement of 18 degree.
\(\therefore\) cos 72 = cos(90 – 18) = sin 18 = \(\sqrt{5} – 1\over 4\)
Hence, cos 72 degrees is \(\sqrt{5} – 1\over 4\).
