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Statement I In a Delta ABC, if cos ^(2)"...

Statement I In a `Delta ABC, if cos ^(2)""A/2 + cos ^(2)""B/2 +cos ^(2) ""C/2=y(x^(2)+(1)/(x^(2)))` then the maximum value of `y is 9/8.`
Statement II In a `Delta ABC, sin ""A/2 . Sin ""B/2 sin ""C/2 le 1/8`

A

Both Statement I and Statement II are correct and Statement II is the correct explanation of Statement I

B

Both Statement I and Statement II are correct and Statement II is not the correct explanation of Statement I

C

Statement I is correct but Statement II is incorrect

D

Statement I is correct but Statement I is incorrect

Text Solution

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The correct Answer is:
To solve the given problem, we will analyze both statements one by one. ### Step 1: Analyze Statement I The first statement is: \[ \cos^2\left(\frac{A}{2}\right) + \cos^2\left(\frac{B}{2}\right) + \cos^2\left(\frac{C}{2}\right) = y\left(x^2 + \frac{1}{x^2}\right) \] We need to find the maximum value of \(y\). #### Step 1.1: Use Known Results From properties of triangles, we know: \[ \cos^2\left(\frac{A}{2}\right) + \cos^2\left(\frac{B}{2}\right) + \cos^2\left(\frac{C}{2}\right) \leq \frac{9}{4} \] This will be our Equation (1). #### Step 1.2: Analyze \(x^2 + \frac{1}{x^2}\) We can express \(x^2 + \frac{1}{x^2}\) as: \[ x^2 + \frac{1}{x^2} = \left(x - \frac{1}{x}\right)^2 + 2 \] This implies that \(x^2 + \frac{1}{x^2} \geq 2\) (Equation 2). #### Step 1.3: Combine Results From Equation (1) and Equation (2), we can substitute: \[ y\left(x^2 + \frac{1}{x^2}\right) \leq \frac{9}{4} \] Substituting the minimum value of \(x^2 + \frac{1}{x^2}\) (which is 2): \[ y \cdot 2 \leq \frac{9}{4} \] This leads to: \[ y \leq \frac{9}{8} \] Thus, the maximum value of \(y\) is \(\frac{9}{8}\). ### Conclusion for Statement I The first statement is correct. --- ### Step 2: Analyze Statement II The second statement is: \[ \sin\left(\frac{A}{2}\right) \cdot \sin\left(\frac{B}{2}\right) \cdot \sin\left(\frac{C}{2}\right) \leq \frac{1}{8} \] We need to prove this statement. #### Step 2.1: Use the AM-GM Inequality By the Arithmetic Mean-Geometric Mean (AM-GM) inequality: \[ \sqrt[3]{\sin^2\left(\frac{A}{2}\right) \cdot \sin^2\left(\frac{B}{2}\right) \cdot \sin^2\left(\frac{C}{2}\right)} \leq \frac{\sin^2\left(\frac{A}{2}\right) + \sin^2\left(\frac{B}{2}\right) + \sin^2\left(\frac{C}{2}\right)}{3} \] #### Step 2.2: Use Known Results We know: \[ \sin^2\left(\frac{A}{2}\right) + \sin^2\left(\frac{B}{2}\right) + \sin^2\left(\frac{C}{2}\right) \geq \frac{3}{4} \] This will be our Equation (3). #### Step 2.3: Substitute in AM-GM Substituting Equation (3) into the AM-GM inequality gives: \[ \sqrt[3]{\sin^2\left(\frac{A}{2}\right) \cdot \sin^2\left(\frac{B}{2}\right) \cdot \sin^2\left(\frac{C}{2}\right)} \leq \frac{\frac{3}{4}}{3} = \frac{1}{4} \] Cubing both sides: \[ \sin^2\left(\frac{A}{2}\right) \cdot \sin^2\left(\frac{B}{2}\right) \cdot \sin^2\left(\frac{C}{2}\right) \leq \left(\frac{1}{4}\right)^3 = \frac{1}{64} \] #### Step 2.4: Relate to Original Statement Taking the square root gives: \[ \sin\left(\frac{A}{2}\right) \cdot \sin\left(\frac{B}{2}\right) \cdot \sin\left(\frac{C}{2}\right) \leq \frac{1}{8} \] ### Conclusion for Statement II The second statement is also correct. ### Final Conclusion Both statements are correct. ---
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