Chapter 10: Problem 9
For Exercises \(1-9,\) find the first four nonzero terms of the Taylor series for the function about 0. $$\ln (5+2 x)$$
Short Answer
Expert verified
The first four nonzero terms are: \( \ln(5) + \frac{2}{5}x - \frac{2}{25}x^2 + \frac{2}{125}x^3 \).
Step by step solution
01
Understand Taylor Series
The Taylor series for a function \( f(x) \) about 0 is given by: \[ f(x) = f(0) + f'(0)x + \frac{f''(0)}{2!}x^2 + \frac{f'''(0)}{3!}x^3 + \cdots \] To find the first four nonzero terms, we need to compute the derivatives of \( \ln(5 + 2x) \) at \( x = 0 \).
02
Evaluate Function at Zero
The first term of the Taylor series is \( f(0) \). For \( f(x) = \ln(5 + 2x) \), at \( x = 0 \), we have: \[ f(0) = \ln(5) \].
03
Calculate First Derivative and Evaluate
Find the first derivative of \( \ln(5 + 2x) \) and evaluate at \( x = 0 \). \[ f'(x) = \frac{2}{5 + 2x} \] Evaluating at \( x = 0 \), we get: \[ f'(0) = \frac{2}{5} \].
04
Calculate Second Derivative and Evaluate
Next, find the second derivative and evaluate at \( x = 0 \). \[ f''(x) = -\frac{4}{(5 + 2x)^2} \] Therefore, \( f''(0) = -\frac{4}{25} \).
05
Calculate Third Derivative and Evaluate
Calculate the third derivative and evaluate at \( x = 0 \). \[ f'''(x) = \frac{16}{(5 + 2x)^3} \] At \( x = 0 \), \( f'''(0) = \frac{16}{125} \).
06
Write First Four Nonzero Terms
Substitute the computed derivatives into the Taylor series formula: \[ \ln(5 + 2x) \approx \ln(5) + \left(\frac{2}{5}\right)x + \left(-\frac{4}{25}\right)\frac{x^2}{2} + \left(\frac{16}{125}\right)\frac{x^3}{6} \] Simplifying, the first four nonzero terms are: \[ \ln(5) + \frac{2}{5}x - \frac{2}{25}x^2 + \frac{2}{125}x^3 \].
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Derivatives
In mathematics, derivatives play a fundamental role. They measure how a function changes as its input changes. Think of it as the slope of a line that touches a point on a curve. In the context of Taylor series, derivatives help us determine the coefficients of the series. For a function like \( \ln(5 + 2x) \), its derivatives at a specific point (like \( x = 0 \)) are crucial for constructing the series.
- The first derivative tells us the slope of the tangent line at the point.
- Higher-order derivatives give the curvature and help define the shape of the polynomial.
- First derivative: \( f'(x) = \frac{2}{5 + 2x} \)
- Second derivative: \( f''(x) = -\frac{4}{(5 + 2x)^2} \)
- Third derivative: \( f'''(x) = \frac{16}{(5 + 2x)^3} \)
ln Function
The linear function, often written as \( \ln(x) \), represents the natural logarithm. This function is unique because it describes the time needed to reach a certain level of growth. In calculus, the ln function is often involved in problems related to growth or decay. For example, \( \ln(5+2x) \) implies a shift in the logarithmic function by a constant and a factor of 2. When working with Taylor series, we aim to approximate \( \ln \) functions using polynomials.
- The natural logarithm \( \ln(5) \) is the starting point of our polynomial approximation.
- The derivatives of \( \ln(5+2x) \) help us understand how this function behaves around \( x = 0 \).
Polynomial Approximation
Polynomial approximation is a powerful technique in calculus. It involves representing a complex function with simpler polynomial terms. The Taylor series is one such method that helps us approximate functions around a specific point, often zero. By using the derivatives of a function, we construct a polynomial that closely follows the original function near the chosen point.
- The first term \( \ln(5) \) is the constant term that represents the value of the function at \( x = 0 \).
- Each subsequent term involves derivatives evaluated at zero, multiplied by \( x \) raised to increasing powers.
- This gives us an expression: \( \ln(5+2x) \approx \ln(5) + \frac{2}{5}x - \frac{2}{25}x^2 + \frac{2}{125}x^3 \)