Question

Find the value of
Cos 36​

Answer 1

Answer:

answer is down

Let A = 18°

Therefore, 5A = 90°

⇒ 2A + 3A = 90˚

⇒ 2θ = 90˚ - 3A

Taking sine on both sides, we get

sin 2A = sin (90˚ - 3A) = cos 3A

⇒ 2 sin A cos A = 4 cos3 A - 3 cos A

⇒ 2 sin A cos A - 4 cos3 A + 3 cos A = 0

⇒ cos A (2 sin A - 4 cos2 A + 3) = 0

Dividing both sides by cos A = cos 18˚ ≠ 0, we get

⇒ 2 sin θ - 4 (1 - sin2 A) + 3 = 0

⇒ 4 sin2 A + 2 sin A - 1 = 0, which is a quadratic in sin A

Therefore, sin θ = −2±−4(4)(−1)√2(4)

⇒ sin θ = −2±4+16√8

⇒ sin θ = −2±25√8

⇒ sin θ = −1±5√4

Now sin 18° is positive, as 18° lies in first quadrant.

Therefore, sin 18° = sin A = −1±5√4

Now, cos 36° = cos 2 ∙ 18°

⇒ cos 36° = 1 - 2 sin2 18°

⇒ cos 36° = 1 - 2(5√−14)2

⇒ cos 36° = 16−2(5+1−25√)16

⇒ cos 36° = 1+45√16

⇒ cos 36° = 5√+14

Therefore, cos 36° = 5√+14

Answer 2

cos 36° = √5+1/4

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