Problem 76

Question

a. Prove that $f(x)=x-\ln x$ is increasing for $x>1$ b. Using part (a), show that $\ln x1$

Step-by-Step Solution

Verified

Answer

a. $f(x)$ is increasing for $x > 1$. b. It follows that $\ln x < x$ for $x > 1$.

1Step 1: Find First Derivative

To determine if the function is increasing, we need to find the first derivative $f'(x)$ of $f(x) = x - \ln x$. The derivative of $x$ is 1 and the derivative of $\ln x$ is $\frac{1}{x}$. Therefore, $f'(x) = 1 - \frac{1}{x}$.

2Step 2: Analyze the Sign of First Derivative

To prove that $f(x)$ is increasing, we need $f'(x) > 0$ for $x > 1$. Since $f'(x) = 1 - \frac{1}{x}$, this simplifies to $1 - \frac{1}{x} > 0$. By rearranging terms, we get $1 > \frac{1}{x}$, or equivalently $x > 1$, which is true for $x > 1$. Thus, $f(x)$ is increasing for $x > 1$.

3Step 3: Relate Increasing Function to Inequality

Since $f(x) = x - \ln x$ is increasing for $x > 1$, it implies that $f(x)$ is greater than its value at any previous point in the interval. In particular, because $f(1) = 0$, for any $x > 1$, $f(x) > 0$ holds.

4Step 4: Conclude the Inequality

Since $f(x) > 0$ for $x > 1$, we have $x - \ln x > 0$, which simplifies to $x > \ln x$. Thus, the inequality $\ln x < x$ is valid for $x > 1$.

Key Concepts

Increasing functionsFirst derivativeLogarithmic inequalityFunction analysis

Increasing functions

An increasing function is a type of function where the value continuously rises as the input increases. For a function $ f(x) $, like $ f(x) = x - \ln x $, this means that as $ x $ gets larger, $ f(x) $ also gets larger. This is verified mathematically by analyzing the first derivative of the function.

If the first derivative $ f'(x) $ is positive over an interval, the function is increasing on that interval.
If $ f'(x) > 0 $ for $ x > 1 $, then $ f(x) $ consistently increases in that range.

Understanding whether a function is increasing is crucial in analysis and solving inequalities, suggesting not only growth but also stability over specified ranges.

First derivative

The first derivative of a function $ f(x) $ gives us the slope of the tangent line at any point on the function's curve. It provides vital information about the behavior of the function:

A positive derivative indicates that the function is increasing at that point.
A negative derivative indicates that the function is decreasing.

For the function $ f(x) = x - \ln x $, the first derivative is calculated as $ f'(x) = 1 - \frac{1}{x} $. This derivative helps determine whether the function is increasing:- If $ x > 1 $, then $ \frac{1}{x} < 1 $, resulting in $ f'(x) > 0 $, so the function increases when $ x $ is greater than 1.

Logarithmic inequality

A logarithmic inequality often involves expressions like $ \ln x < x $. To validate this inequality for $ x > 1 $, we exploit the function $ f(x) = x - \ln x $. Since $ f(x) $ is shown to increase when $ x > 1 $, we can infer certain properties:

When $ x = 1 $, $ f(x) = 0 $.
For any $ x > 1 $, $ f(x) > 0 $.

Thus, for $ x > 1 $, $ x - \ln x > 0 $, hence confirming the inequality $ \ln x < x $. This relationship is pivotal in understanding how exponentials relate to their logarithmic counterparts, proving useful across various mathematical models and real-world applications.

Function analysis

Function analysis is the comprehensive study of the properties of functions through tools like derivatives and critical points. For a function $ f(x) $, this analysis becomes essential in understanding its overall behavior:

Identifying where the function increases or decreases.
Determining key points such as local maxima or minima.

When analyzing $ f(x) = x - \ln x $, the process involves finding $ f'(x) $ and examining its sign. Since $ f'(x) = 1 - \frac{1}{x} $ is positive for $ x > 1 $, it indicates $ f(x) $ will continue to increase beyond this point.Such analysis helps ascertain the validity of logarithmic inequalities and better understand the characteristics of mathematical relationships. This approach extends to numerous functions and real-life problems, aiding in constructing models and predictions.

Problem 76

Other exercises in this chapter

Problem 76

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