\((1 / 2)+(3 / 4)+(7 / 8)+(15 / 16) \ldots 100\) terms \(=\) (A) \(2^{100}+99\) (B) \(2^{-100}+99\) (C) \(2^{-101}+100\) (D) \(2^{-99}+99\)

Short Answer

Expert verified
The short answer is \(\boxed{(D)}\), since the sum of the given series is \(\left(\frac{1}{2}\right)^{99} + 99\).

Step by step solution

01

Identify the series pattern

Before finding the sum of the series, let's look at the pattern of the given series. The series is: \[ \frac{1}{2} + \frac{3}{4} + \frac{7}{8} + \frac{15}{16} \ldots \text{[100 terms]} \] Notice that the series can be written as: \[ \frac{2^1 - 1}{2^1} + \frac{2^2 - 1}{2^2} + \frac{2^3 - 1}{2^3} + \frac{2^4 - 1}{2^4} \ldots \]
02

Observe the cancellation pattern

Observe that the numerators have a power of 2 minus 1 pattern, so when you add consecutive terms, there'll be a cancellation effect taking place. Let's see what happens when we add the first term to the second term and so on: \[ \left(1 - \frac{1}{2}\right) + \left(1 - \frac{1}{4}\right) + \left(1 - \frac{1}{8}\right) + \left(1 - \frac{1}{16}\right) \ldots \]
03

Find the sum of the series

From the pattern above, we can add the consecutive terms so that the series becomes the sum of the \(100\) terms minus the sum of the fractions \(1/2, 1/4, 1/8, ...\) up to the \(99th\) term. Now let's find the sum of the fractions, which is a geometric series with the common ratio of \(1/2\). The sum of an infinite geometric series is given by the formula \(\frac{a}{1 - r}\), where \(a\) is the first term and \(r\) is the common ratio. However, since we want the sum of the first 99 terms, the formula will be \(\frac{a(1 - r^n)}{1 - r}\), where n is the number of terms. Sum of fractions: \[ S = \frac{\frac{1}{2}(1 - \left(\frac{1}{2}\right)^{99})}{1 - \frac{1}{2}} = 1 - \left(\frac{1}{2}\right)^{99} \] So, the sum of the given series is: Sum of the given series = \(100\) - Sum of fractions \[ 100 - ( 1 - \left(\frac{1}{2}\right)^{99} ) = \left(\frac{1}{2}\right)^{99} + 99 \] Thus, the sum of the series is: \[ \left(\frac{1}{2}\right)^{99} + 99 \] Comparing the result to the given options, the sum of the series matches with option (D). Answer: \(\boxed{(D)}\).

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