Simple Limit - Calculus

Level 1

What Is The Value of

lim n k = 0 N ( 1 2 ) k \lim_{n \to \infty} \sum_{k=0}^{N}( \frac{1}{2})^{k}


The answer is 2.

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3 solutions

Caleb Stanford
Jan 28, 2014

What a boring question.

Tunk-Fey Ariawan
Jan 29, 2014

There's a bit of typo in the question. It should be lim N k = 0 N ( 1 2 ) k \lim\limits_{N \to \infty} \sum\limits_{k=0}^N \left(\frac{1}{2}\right)^k . It can easily be solved because the series forms a geometric progression. The solution is lim N k = 0 N ( 1 2 ) k = 1 + 1 2 + 1 4 + = 1 1 1 2 = 2 . \lim\limits_{N \to \infty} \sum\limits_{k=0}^N \left(\frac{1}{2}\right)^k = 1 + \frac{1}{2} + \frac{1}{4} + \cdots = \frac{1}{1-\frac{1}{2}} = \boxed{2}. # Q . E . D . # \text{\# }\mathbb{Q}.\mathbb{E}.\mathbb{D}.\text{\#}

Finn Hulse
Jan 28, 2014

First let's just take a look at what's going on here. Plugging in zero, we get 1. From there on out, it's just the harmonic series, which we know sums up to 1. And the most tricky part: 1 + 1 = 2.

You mean, geometric series with common ratio 1 / 2 1/2 . The harmonic series is 1 + 1 / 2 + 1 / 3 + 1 / 4 + 1 / 5 + 1 + 1/2 + 1/3 + 1/4 + 1/5 + \cdots , which diverges.

Caleb Stanford - 7 years, 4 months ago

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