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The value of cot18^(@)(cot72^(@)cos^(2)2...

The value of `cot18^(@)(cot72^(@)cos^(2)22^(@)+(1)/(tan72^(@)sec^(2)68^(@)))` is

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To solve the expression \( \cot 18^\circ \left( \cot 72^\circ \cos^2 22^\circ + \frac{1}{\tan 72^\circ \sec^2 68^\circ} \right) \), we will break it down step by step. ### Step 1: Rewrite the expression We start with the expression: \[ \cot 18^\circ \left( \cot 72^\circ \cos^2 22^\circ + \frac{1}{\tan 72^\circ \sec^2 68^\circ} \right) \] ### Step 2: Simplify the second term Recall that: - \( \tan \theta = \frac{1}{\cot \theta} \) - \( \sec^2 \theta = \frac{1}{\cos^2 \theta} \) Thus, we can rewrite the second term: \[ \frac{1}{\tan 72^\circ \sec^2 68^\circ} = \frac{1}{\tan 72^\circ} \cdot \cos^2 68^\circ \] Since \( \sec^2 68^\circ = \frac{1}{\cos^2 68^\circ} \). ### Step 3: Substitute and combine Now, substituting back into the expression, we have: \[ \cot 18^\circ \left( \cot 72^\circ \cos^2 22^\circ + \cot 72^\circ \cos^2 68^\circ \right) \] Factoring out \( \cot 72^\circ \): \[ \cot 18^\circ \cot 72^\circ \left( \cos^2 22^\circ + \cos^2 68^\circ \right) \] ### Step 4: Use the identity for cosine Using the identity \( \cos(90^\circ - \theta) = \sin \theta \): \[ \cos^2 68^\circ = \cos^2(90^\circ - 22^\circ) = \sin^2 22^\circ \] Thus, we can rewrite: \[ \cos^2 22^\circ + \cos^2 68^\circ = \cos^2 22^\circ + \sin^2 22^\circ = 1 \] ### Step 5: Substitute back into the expression Now substituting this back, we get: \[ \cot 18^\circ \cot 72^\circ \cdot 1 = \cot 18^\circ \cot 72^\circ \] ### Step 6: Use the cotangent identity Recall that: \[ \cot 72^\circ = \tan(90^\circ - 72^\circ) = \tan 18^\circ \] Thus: \[ \cot 18^\circ \cot 72^\circ = \cot 18^\circ \tan 18^\circ \] Since \( \cot \theta = \frac{1}{\tan \theta} \), we have: \[ \cot 18^\circ \tan 18^\circ = 1 \] ### Final Result Thus, the value of the original expression is: \[ \boxed{1} \]
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