We will learn to find the exact value of cos 72 degrees using the formula of submultiple angles.
How to find the exact value of cos 72°?
Let, A = 18°
Therefore, 5A = 90°
⇒ 2A + 3A = 90˚
⇒ 2A = 90˚  3A
Taking sine on both sides, we get
sin 2A = sin (90˚  3A) = cos 3A
⇒ 2 sin A cos A = 4 cos\(^{3}\) A  3 cos A
⇒ 2 sin A cos A  4 cos\(^{3}\) A + 3 cos A = 0
⇒ cos A (2 sin A  4 cos\(^{2}\) A + 3) = 0
Dividing both sides by cos A = cos 18˚ ≠ 0, we get
⇒ 2 sin A  4 (1  sin\(^{2}\) A) + 3 = 0
⇒ 4 sin\(^{2}\) A + 2 sin A  1 = 0, which is a quadratic in sin A
Therefore, sin A = \(\frac{2 \pm \sqrt{ 4 (4)(1)}}{2(4)}\)
⇒ sin A = \(\frac{2 \pm \sqrt{4 + 16}}{8}\)
⇒ sin A = \(\frac{2 \pm 2 \sqrt{5}}{8}\)
⇒ sin A = \(\frac{1 \pm \sqrt{5}}{4}\)
sin 18° is positive, as 18° lies in first quadrant.
Therefore, sin 18° = sin A = \(\frac{√5  1}{4}\)
Now, cos 72° = cos (90°  18°) = sin 18° = \(\frac{√5  1}{4}\)
11 and 12 Grade Math
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