What is the Value of Cos 72 Degrees ?

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 \(\sqrt{5} – 1\over 4\).

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