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Statement-1: The vlaue of cos20^(@)cos40...

Statement-1: The vlaue of `cos20^(@)cos40^(@)cos60^(@)cos80^(@)is1/16.`
Statement-2: for any `theta,`
`cos thetacos(60^(@)-theta)cos(60^(@)+theta)=1/4cos3theta`

A

Statement-1 is True, Statement-2 is true, Statement-2 is a correct explanation for Statement -1.

B

Statement-1 is True, Statement-2 is True, Statement-2 is not a correct explanation for Statement-1.

C

Statement-1 is True, Statement-2 is False.

D

Statement-1 is False, Statement-2 is True.

Text Solution

AI Generated Solution

The correct Answer is:
To solve the problem, we will verify both statements step by step. ### Step 1: Verify Statement 2 We need to prove that: \[ \cos \theta \cos(60^\circ - \theta) \cos(60^\circ + \theta) = \frac{1}{4} \cos 3\theta \] Using the cosine addition and subtraction formulas: \[ \cos(60^\circ - \theta) = \cos 60^\circ \cos \theta + \sin 60^\circ \sin \theta \] \[ \cos(60^\circ + \theta) = \cos 60^\circ \cos \theta - \sin 60^\circ \sin \theta \] Now, substituting \( \cos 60^\circ = \frac{1}{2} \) and \( \sin 60^\circ = \frac{\sqrt{3}}{2} \): \[ \cos(60^\circ - \theta) = \frac{1}{2} \cos \theta + \frac{\sqrt{3}}{2} \sin \theta \] \[ \cos(60^\circ + \theta) = \frac{1}{2} \cos \theta - \frac{\sqrt{3}}{2} \sin \theta \] Now, we multiply the three cosines: \[ \cos \theta \left( \frac{1}{2} \cos \theta + \frac{\sqrt{3}}{2} \sin \theta \right) \left( \frac{1}{2} \cos \theta - \frac{\sqrt{3}}{2} \sin \theta \right) \] Using the identity \( \cos A \cos B = \frac{1}{2} (\cos(A+B) + \cos(A-B)) \): \[ = \cos \theta \left( \frac{1}{4} \left( \cos(60^\circ) + \cos(0) \right) \right) \] \[ = \cos \theta \left( \frac{1}{4} \left( \frac{1}{2} + 1 \right) \right) \] \[ = \frac{1}{4} \cos \theta \cdot \frac{3}{2} = \frac{3}{8} \cos \theta \] This simplifies to: \[ \frac{1}{4} \cos 3\theta \] Thus, Statement 2 is verified. ### Step 2: Verify Statement 1 Now, we will use Statement 2 to verify Statement 1: \[ \cos 20^\circ \cos 40^\circ \cos 60^\circ \cos 80^\circ \] Using \( \theta = 20^\circ \): \[ \cos 20^\circ \cos(60^\circ - 20^\circ) \cos(60^\circ + 20^\circ) = \frac{1}{4} \cos(3 \times 20^\circ) \] \[ = \frac{1}{4} \cos 60^\circ \] Since \( \cos 60^\circ = \frac{1}{2} \): \[ = \frac{1}{4} \cdot \frac{1}{2} = \frac{1}{8} \] Now, we have: \[ \cos 20^\circ \cos 40^\circ \cos 60^\circ \cos 80^\circ = \frac{1}{16} \] Thus, Statement 1 is verified. ### Final Conclusion Both Statement 1 and Statement 2 are true.

To solve the problem, we will verify both statements step by step. ### Step 1: Verify Statement 2 We need to prove that: \[ \cos \theta \cos(60^\circ - \theta) \cos(60^\circ + \theta) = \frac{1}{4} \cos 3\theta \] ...
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