Let $\log_2 n$ be an integer. If $\prod_{k=1}^{\log_2 n}\left(x^{\frac{n}{2^k}}+1\right) = \dfrac{x^4 - B}{x - C}$, where $A$, $B$ and $C$ are positive integers. Then the value of $(B + C + \log_2 A)$ for $n = 2^{92}$ is:
Step-by-Step Solution
Key Concept: Telescoping product using the identity $(x-1)\prod_{j=0}^{m-1}(x^{2^j}+1) = x^{2^m}-1$.
Step 1: Identify the telescoping product identity.
We recognize that the product can be simplified using the algebraic identity:
$$(x^m - 1)(x^m + 1) = x^{2m} - 1$$
This suggests that a telescoping product will emerge when we multiply terms of the form $(x^{2^j} + 1)$.
Step 2: Rewrite the product in terms of powers of 2.
Let $m = \log_2 n$, so $n = 2^m$.
The given product becomes:
$$\prod_{k=1}^{m}\left(x^{\frac{2^m}{2^k}}+1\right) = \prod_{k=1}^{m}\left(x^{2^{m-k}}+1\right)$$
Step 3: Change the index of summation.
Let $j = m - k$. As $k$ ranges from 1 to $m$, the index $j$ ranges from $m-1$ to 0:
$$\prod_{k=1}^{m}\left(x^{2^{m-k}}+1\right) = \prod_{j=0}^{m-1}\left(x^{2^j}+1\right)$$
Step 4: Apply the telescoping product formula.
We use the fundamental identity:
$$(x-1)\prod_{j=0}^{m-1}\left(x^{2^j}+1\right) = x^{2^m}-1$$
Solving for the product:
$$\prod_{j=0}^{m-1}\left(x^{2^j}+1\right) = \frac{x^{2^m}-1}{x-1}$$
Since $n = 2^m$, we have $2^m = n$:
$$\prod_{j=0}^{m-1}\left(x^{2^j}+1\right) = \frac{x^n - 1}{x-1}$$
Step 5: Substitute $n = 2^{92}$.
For $n = 2^{92}$, we have $m = 92$, so:
$$\prod_{k=1}^{92}\left(x^{\frac{2^{92}}{2^k}}+1\right) = \frac{x^{2^{92}}-1}{x-1}$$
Step 6: Match with the given form.
The problem states that the product equals $\dfrac{x^A - B}{x - C}$.
Comparing $\dfrac{x^{2^{92}}-1}{x-1}$ with $\dfrac{x^A - B}{x - C}$:
$$A = 2^{92}, \quad B = 1, \quad C = 1$$
Step 7: Calculate the final answer.
$$B + C + \log_2 A = 1 + 1 + \log_2(2^{92}) = 1 + 1 + 92 = 94$$
**Final Answer: 94**
The answer is **Option 3: 94**
Correct Answer: 3