3
$\begingroup$

What is the pattern in the following?

  • Sum to $n$ terms of the series: $ \frac{1}{2} + \frac{3}{4} + \frac{7}{8} + \frac{15}{16} + \cdots$
  • 5
    Hint: The $k$th term is $1-\frac1{2^k}$.2012-06-03

3 Answers 3

14

Hint:

Write it as $(1-{1\over2})+(1-{1\over4})+(1-{1\over 8})+\cdots+(1-{1\over 2^n}).$

5

Here is the pattern:

\begin{align*} \frac{1}{2} + \frac{3}{4} + \cdots &= \biggl(1-\frac{1}{2}\biggr) + \biggl(1-\frac{1}{2^2}\biggr) + \cdots \\\ &= (1+1+\cdots + 1) - \biggl(\frac{1}{2}+\frac{1}{2^2} + \cdots +\frac{1}{2^n}\biggr) \end{align*}

0

So, what you want is basically $\left(1 - \frac {1} {2}\right) + \left(1 - \frac {1} {4}\right) + \cdots + \left(1 - \frac {1} {2^n}\right) = n - \left(\frac {1} {2} + \frac {1} {4} + \cdots + \frac {1} {2^n}\right) = n - 1 + \frac {1} {2^n}.$