Problem 58 - Contemporary Precalculus

Question

Sketch the graph of the function. $$g(x)=2 \ln x+3$$

Step-by-Step Solution

Answer

Short Answer: To sketch the graph of $$g(x)=2\ln x+3$$, start with the parent function $$f(x)=\ln x$$, and apply a vertical stretch of 2 and a vertical shift upwards of 3. The domain of the function is $$(0, +\infty)$$, and the range is $$(-\infty,+\infty)$$. Plot key points (1,3) and $$(e,5)$$ on the graph, draw a vertical asymptote at $$x=0$$, and then connect the points with the appropriate curve for the logarithmic function.

Step 1: Identify Parent Function and Transformations

The given function $$g(x)=2\ln x+3$$ has a parent function of $$f(x)=\ln x$$. The coefficient 2 in front of the natural logarithm causes a vertical stretch, and the constant 3 added at the end results in a vertical shift upwards.

Step 2: Determine the Domain and Range

The natural logarithm function is only defined for positive inputs (x > 0). Therefore, the domain of the function is $$(0,+\infty)$$. The range of the base logarithmic function $$f(x)=\ln x$$ is $$(-\infty,+\infty)$$, and the coefficient 2 and constant 3 do not affect the range. So, the range of our function $$g(x)=2\ln x+3$$ is also $$(-\infty,+\infty)$$.

Step 3: Identify Key Points

Key points on the base logarithmic function $$f(x)=\ln x$$ include (1,0) and $$(e,1)$$. Since the function $$g(x)=2\ln x + 3$$ is vertically stretched and shifted upwards by 2 and 3 respectively, we need to apply these transformations to the key points. The new key points of $$g(x)$$ are: (1,3) : 2*0 + 3 = 3 $$(e,5)$$ : 2*1 + 3 = 5

Step 4: Sketch the Graph

Begin graphing by sketching the parent function $$f(x)=\ln x$$. Draw a vertical asymptote along the y-axis, $$x = 0$$, since the logarithmic function approaches but never touches the y-axis. Next, plot the key points we found in Step 3, (1,3) and $$(e,5)$$, on the graph. Now apply the vertical stretch of 2 and the vertical shift upwards of 3 to the base function. The new graph should show the function $$g(x)=2\ln x+3$$ with the identified transformations and key points. The final step is to check if the graph appropriately represents the function, domain, and range.

Key Concepts

Natural Logarithm

The natural logarithm, denoted as \( f(x) = \ln x \), is a special kind of logarithm that uses Euler's number (approximately 2.718) as its base. It's an important function in calculus and mathematical analysis due to its natural properties and its appearance in exponential growth and decay problems.

• It is the inverse of the exponential function \( e^x \).
• For any positive number \( x \), \( \ln x \) represents the power to which \( e \) must be raised to result in \( x \).

One essential characteristic of the natural logarithm is that it is only defined for positive real numbers, which is crucial when determining the domain and range of functions involving \( \ln x \). It helps us understand transformations and movements within a graph when altering this base logarithmic function.

Vertical Transformation

Vertical transformations are changes made to the vertical positioning of a function on a graph - either stretching, shrinking, or shifting it. These are common modifications that can be seen in the transformed function \( g(x) = 2\ln x + 3 \).
- **Vertical Stretch**: In \( g(x) \), the coefficient 2 in front of \( \ln x \) indicates a vertical stretch. This means each output from the parent function \( \ln x \) is multiplied by 2, making the graph steeper.
- **Vertical Shift**: The constant +3 causes the graph to shift upwards by 3 units. This affects each point on the graph by adding 3 to the \( y \)-value.
Vertical transformations do not alter the domain of the function, but they significantly change the appearance and position of the graph.

Domain and Range

Understanding the domain and range is key to graphing functions accurately. The domain refers to all possible input values, while the range refers to potential output values.
- **Domain**: For \( g(x) = 2\ln x + 3 \), the domain is defined by the natural logarithm. Since \( \ln x \) is only defined for \( x > 0 \), the domain of \( g(x) \) is \( (0, +\infty) \).
- **Range**: The range of the natural logarithm \( f(x) = \ln x \) is \( (-\infty, +\infty) \), since \( \ln x \) can take any real value. Subsequently, multiplying by 2 and adding 3 doesn't modify this property, so the range of \( g(x) \) remains \( (-\infty, +\infty) \).
Considering the domain and range helps ensure that the graph accurately reflects the behavior of the function across all possible inputs.

Parent Function

The parent function serves as the fundamental structure from which transformations are applied to create new functions. In the case of \( g(x) = 2 \ln x + 3 \), the parent function is \( f(x) = \ln x \).

• **Key Characteristics**: The basic shape and behavior of \( \ln x \) include a rapid increase in the positive direction and the characteristic vertical asymptote at \( x = 0 \).
• **Transformations**: Alterations like vertical stretching or shifting lead to changes in the parent's graph, forming a new function with attributes derived from \( \ln x \).

By starting with a parent function, it becomes straightforward to apply various transformations like multiplying by a constant or adding a value. This reiterates the function's underlying properties and simplifies understanding complex graphs.