- A
- B
- C
- D
View written solutionFree
Correct answer: B
The user wants me to solve the following trigonometric expression:
Step 1: Simplify the inner sum. First, we evaluate the summation inside the argument of the inverse cotangent function:
Step 2: Rewrite the expression. Substitute the simplified sum back into the main expression. The expression becomes:
Step 3: Convert to inverse tangent. We use the identity for . Since , the term is always positive. The general term of the summation can be written as: {\cot }^{ - 1}} \left( {1 + n(n+1)} \right) = {\tan }^{ - 1}} \left( \frac{1}{1 + n(n+1)} \right)
Step 4: Use the telescopic series identity for inverse tangent. We use the identity . We can rewrite the argument of the inverse tangent as: This fits the identity with and . So, we have: {\tan }^{ - 1}} \left( \frac{1}{1 + n(n+1)} \right) = \tan^{-1}(n+1) - \tan^{-1}(n)
Step 5: Evaluate the summation. The summation becomes a telescoping series: Let's write out the first few and the last term of the series: For : For : For : ... For :
Summing these terms, all intermediate terms cancel out:
Step 6: Simplify the sum. Using the identity again:
Step 7: Calculate the final value. The original expression is . Let . This implies . Since , we have:
Thus, the value of the given expression is . This corresponds to option B.
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