The Exact Value of sin 18 degrees (With Figure-Solution)

One of the most important and essential exact value in trigonometry, sin 18 degrees. When the exact value of this is already known, other exact values can be solved by using different formulas.

By the aid of a right triangle with interior angles: 36deg, 90deg, 54deg, we can use the Functions of Complementary Angles Formula cosA = cos(90deg-B) = sinB, in which we can find a quadratic equation. By employing the Quadratic Formula, we can then find the exact value of sin 18 degrees = (√5-1)/4.

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