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Exact Value of cos 27°

We will learn to find the exact value of cos 27 degrees using the formula of submultiple angles.


How to find the exact value of cos 27°?

Solution: 

We have, (sin 27° + cos 27°)2 = sin2 27° + cos2 27° + 2 sin 27° cos 27°

⇒ (sin 27° + cos 27°)2 = 1 + sin 2 ∙ 27°

⇒ (sin 27° + cos 27°)2 = 1 + sin 54° 

⇒ (sin 27° + cos 27°)2 = 1 + sin (90° - 36°)

⇒ (sin 27° + cos 27°)2 = 1 + cos 36° 

⇒ (sin 27° + cos 27°)2 = 1+ 5+14

⇒ (sin 27° + cos 27°)2 = 14 ( 5 + √ 5)

Therefore,  sin 27° + cos 27° = 125+5 …………….….(i)

[Since, sin 27° > 0 and cos 27° > 0)

Similarly, we have, 

(sin 27° - cos 27°)2 = 1 - cos 36°

⇒ (sin 27° - cos 27°)2 = 1 - 5+14

⇒ (sin 27° - cos 27°)2 = 14 (3 - √5  )

Therefore, sin 27° - cos 27° = ± 1235 …………….….(ii)

Now, sin 27° - cos 27° = √2 (12 sin 27˚ - 12 cos 27°)

                               = √2 (cos 45° sin 27° - sin 45° cos 27°)

                               = √2 sin (27° - 45°)

                               = -√2 sin 18° < 0

Therefore, from (ii) we get,

sin 27° - cos 27° = -1235 …………….….(iii)

Now, subtracting (iii) and (i) we get,

2 cos 27° = 125+5 + 1235

⇒ cos 27° = 14(5+5+35)

Therefore, cos 27° = 14(5+5+35)      

 Submultiple Angles





11 and 12 Grade Math

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