Master Calculus! Get instant help on “I Aced Calculus AP” App. Hundreds of flashcards and practice questions at your fingertips. Download now on the App Store and Google Play.
7 Nov 2024
The antiderivative of \( \dfrac{1}{x} \) is a foundational result in calculus, and appears in a variety of contexts. Specifically, it reveals interesting properties of the natural logarithm in calculus.
The antiderivative of \( \dfrac{1}{x} \) with respect to \( x \) is:
\[\begin{align*} \displaystyle \int \dfrac{1}{x} \, dx = \ln |x| + C \end{align*}\]
where \( C \) is the constant of integration. The result follows from the fact that the derivative of \( \ln |x| \) is \( \dfrac{1}{x} \). This is a key relationship in calculus.
We use \( |x| \) to cover both positive and negative \( x \) values. This ensures the result is defined for all \( x \neq 0 \), as \( \dfrac{1}{x} \) is undefined at \( x = 0 \).
This integral shows up in calculations in fields such as:
Problem 1: Compute \( \displaystyle \int \dfrac{1}{3x} \, dx \).
Solution: To integrate \( \dfrac{1}{3x} \), first factor out the constant \( \dfrac{1}{3} \):
\[\begin{align*} \displaystyle \int \dfrac{1}{3x} \, dx = \dfrac{1}{3} \displaystyle \int \dfrac{1}{x} \, dx. \end{align*}\]
Applying the antiderivative of \( \dfrac{1}{x} \), we get:
\[\begin{align*} \dfrac{1}{3} \ln |x| + C. \end{align*}\]
Thus, the solution is \( \dfrac{1}{3} \ln |x| + C \).
Problem 2: Find \( \displaystyle \int \dfrac{2}{x} \, dx \).
Solution: Again, factor out the constant \( 2 \):
\[\begin{align*} \displaystyle \int \dfrac{2}{x} \, dx = 2 \displaystyle \int \dfrac{1}{x} \, dx. \end{align*}\]
Using the antiderivative of \( \dfrac{1}{x} \):
\[\begin{align*} 2 \ln |x| + C. \end{align*}\]
So, the result is \( 2 \ln |x| + C \).
Problem 3: Evaluate \( \displaystyle \int_2^6 \dfrac{1}{x} \, dx \).
Solution: Here, we need the definite integral from \( x = 2 \) to \( x = 6 \):
\[\begin{align*} \displaystyle \int_2^6 \dfrac{1}{x} \, dx = \ln |x|\bigg|_2^6. \end{align*}\]
Substitute the limits:
\[\begin{align*} \ln |6| – \ln |2| = \ln 6 – \ln 2 = \ln \dfrac{6}{2} = \ln 3. \end{align*}\]
Thus, the answer is \( \ln 3 \), which represents the total change in logarithmic terms over this interval.