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If cot A = 13\frac{1}{\sqrt3}3​1​​, then what is the value of ​(1−cos 2A)(1+cos 2A)?\frac{\lparen1-cos\space2A\rparen}{\lparen1+cos\spa
Question

If cot A = 13\frac{1}{\sqrt3}​, then what is the value of ​(1cos 2A)(1+cos 2A)?\frac{\lparen1-cos\space2A\rparen}{\lparen1+cos\space2A\rparen}?

​​​

A.

1.5

B.

3\sqrt3​​

C.

13\frac{1}{3}​​

D.

3

Correct option is D

Given:

cotA=13\cot A = \frac{1}{\sqrt{3}}​​

We need to determine the value of:

1cos2A1+cos2A\frac{1 - \cos 2A}{1 + \cos 2A}​​

Concept Used:

Using the identity:

cos2A=1tan2A1+tan2A\cos 2A = \frac{1 - \tan^2 A}{1 + \tan^2 A}​​

1cos2A1+cos2A=tan2A\frac{1 - \cos 2A}{1 + \cos 2A} = \tan^2A

Solution:

tanA=1cotA=3\tan A = \frac{1}{\cot A} = \sqrt{3}​​

tan2A=3\tan^2 A = 3 

So, 

1cos2A1+cos2A=tan2A=3\frac{1 - \cos 2A}{1 + \cos 2A} = \tan^2 A =3​​

Thus, correct answer is option (D).

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