Math Problem Statement

Calculate the limit of the expression (x + x^2 + ... + x^n - n) / ((2 - x)^n - 1) as x approaches 1.

Solution

The limit expression you provided is:

limx1x+x2++xnn(2x)n1\lim_{{x \to 1}} \frac{x + x^2 + \cdots + x^n - n}{(2 - x)^n - 1}

Let's break this down and evaluate the limit as xx approaches 1.

Step-by-Step Evaluation:

1. Simplify the Numerator:

The numerator is a sum of powers of xx, specifically: x+x2++xnnx + x^2 + \cdots + x^n - n This is a finite geometric series, excluding the subtraction of nn. The sum of the geometric series can be written as: S=x+x2++xn=x(1xn)1xS = x + x^2 + \cdots + x^n = \frac{x(1 - x^n)}{1 - x} Subtracting nn gives: Sn=x(1xn)1xnS - n = \frac{x(1 - x^n)}{1 - x} - n

2. Simplify the Denominator:

The denominator is: (2x)n1(2 - x)^n - 1 At x=1x = 1, this expression becomes (1)n1=0(1)^n - 1 = 0. So, we will need to use L'Hôpital's Rule because both the numerator and denominator tend to 0 as x1x \to 1.

3. Apply L'Hôpital's Rule:

To resolve the indeterminate form, we differentiate the numerator and the denominator with respect to xx.

Differentiation:

  • For the numerator, differentiate the sum term by term.
  • For the denominator, apply the chain rule to differentiate (2x)n1(2 - x)^n - 1.

Let me compute this to confirm the limit evaluation.

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Math Problem Analysis

Mathematical Concepts

Calculus
Limits
Geometric Series

Formulas

Sum of Geometric Series: S = x + x^2 + ... + x^n = x(1 - x^n)/(1 - x)
L'Hôpital's Rule

Theorems

L'Hôpital's Rule

Suitable Grade Level

College