The Exact Value of cos 72 degrees.

The exact value of cos 72deg = (√5-1)/4 can be solve by explicitly isolating cos theta from the sine function's half-angle formula sin (theta/2) = √{[1 - (cos theta)]/2} to cos theta = 1-2[sin (theta/2)]^2, where theta = 72 deg or cos (72deg) = 1-2[sin (72deg/2)]^2 = 1-2[sin (36deg)]^2. Substitute the exact value of sin (36deg) = √(10-2√5)/4 to the formula, in which I've already solved above and included in this playlist. Evaluate and simplify.

Mharthy's Channel's Playlists:

Differential Calculus    • Prove f'(x) = nx^(n-1), if f(x) = x^n...  

Complex Numbers    • Prove:⁡⁡  (a⁡+⁡b⁡i)(c⁡+⁡d⁡i)⁡ = ⁡(ac⁡...  

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Analytic Geometry    • Analytic Geometry  

Plane Trigonometry Basics    • Plane Trigonometry Basics  

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Division of Polynomials, etc.    • Division of Polynomials, etc.  

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Trigonometric Formulas    • Trigonometric Formulas  

The Exact Values of sin & cos Functions of a Right Triangle    • The Exact Values of sin and cos Funct...  

Trigonometric Identities 1    • Trigonometric Identities 1  

Trigonometric Identities 2    • Trigonometric Identities 2  

Trigonometric Identities 3    • Trigonometric Identities 3