## Prove

## or, equivalently,

Prove that:

(that is, without computation or tables).

Hint 1. Find closed form expressions forSee

Sublime Triangle

Closed Form for Some Values of Cosine and Sine

The hints below may be useful for finding closed form expressions OR for approaching the problem algebraically.

Hint 2. Consider the isosceles triangle with angles 36, 72, 72.See

Sublime Triangle.

Hint 3. Consider trigonometric identities for half angles and double angles.

Hint 4. Consider the equation

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