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We will learn to find the exact value of tan 72 degrees using the formula of submultiple angles.
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 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 A - 4 (1 - sin2 A) + 3 = 0
⇒ 4 sin2 A + 2 sin A - 1 = 0, which is a quadratic in sin A
Therefore, sin A = −2±√−4(4)(−1)2(4)
⇒ sin A = −2±√4+168
⇒ sin A = −2±2√58
⇒ sin A = −1±√54
Now sin 18° is positive, as 18° lies in first quadrant.
Therefore, sin 18° = sin A = √5−14
Now, cos 72° = cos (90° - 18°) = sin 18° = √5−14
And cos 18° = √(1 - sin2 18°), [Taking positive value, cos 18° > 0]
⇒ cos 18° = √1−(√5−14)2
⇒ cos 18° = √16−(5+1−2√5)16
⇒ cos 18° = √10+2√516
Thus, sin 72° = sin (90° - 18°) = cos 18° = √10+2√54
Now, tan 72° = sin72°cos72° = √10+2√54√5−14 = √10+2√5√5−1
Therefore, tan 72° =√10+2√5√5−1
11 and 12 Grade Math
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