Correct Answer :
Solution :
The correct answer is 3.
Let's analyze the given expression for :
Notice that this sum is not a standard telescoping series because the terms are skipped: , then , and so on, up to .
Let us write the general term as:
for .
To simplify each term, we can multiply and divide by :
Since , we have:
Using the identity :
Thus, we can write as:
This sum expands to:
Since , we can group the terms from both ends.
Specifically, let us pair the terms:
- The last negative term is which cancels out with the first negative term .
- The last positive term is which cancels out with the second positive term .
In general, for each term in the sum:
Let . The corresponding paired term from the end is:
When we add these up, all terms of the form for cancel in pairs because:
cancels with .
cancels with for all (since , which is not in the set of indices, the term remains unpaired and does not cancel).
Thus, the only term that does not cancel is the very first term:
We know that:
Substituting this back into the expression for :
We want to find the value of:
Thus, the final value is 3.
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