Chapter 4: Problem 28
Suppose that \(f(0)=5\) and that \(f^{\prime}(x)=2\) for all \(x .\) Must \(f(x)=\) \(2 x+5\) for all \(x ?\) Give reasons for your answer.
Short Answer
Expert verified
Yes, \(f(x) = 2x + 5\) since it matches the given derivative and point condition.
Step by step solution
01
Understand the Derivative Condition
The derivative of the function, \(f'(x) = 2\), indicates that the function has a constant rate of change. This means that for every unit increase in \(x\), the function \(f(x)\) increases by 2 units. Therefore, \(f'(x) = 2\) suggests that \(f(x)\) is a linear function with a slope of 2.
02
Identify the General Form of the Function
Since \(f'(x)\) is constant, \(f(x)\) is a linear function of the form \(f(x) = mx + b\). With a slope \(m = 2\), the function becomes \(f(x) = 2x + b\).
03
Use the Given Value \(f(0)=5\)
We know that \(f(0) = 5\). Substitute \(x = 0\) into the function \(f(x) = 2x + b\) to find the intercept \(b\):\[f(0) = 2(0) + b = 5\]Thus, \(b = 5\).
04
Formulate the Specific Function
Now that we have determined \(b = 5\), substitute this back into the equation \(f(x) = 2x + b\):\[f(x) = 2x + 5\]This is the specific form of the function \(f(x)\) given the initial conditions.
05
Verify Consistency
Verify that \(f(x) = 2x + 5\) satisfies both the initial value and the derivative condition:- Plugging \(x = 0\) into \(f(x)\), we get \(f(0) = 2 \cdot 0 + 5 = 5\), consistent with the given \(f(0) = 5\).- The derivative of \(f(x) = 2x + 5\) is \(f'(x) = 2\), consistent with the given \(f'(x) = 2\).This confirms that the function meets all given criteria.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Derivative
In the world of calculus, a derivative is like the ultimate detective. It tells us how a function is changing at any point. It's a measure of a function's sensitivity to change in its input. When we say the derivative of a function is constant, like in our exercise where \(f'(x) = 2\), we're describing a continuous, steady change.
Referring to a constant rate of change, a constant derivative suggests that the function changes by a fixed amount every time its input changes by one unit. This is typical of linear functions, where the graph is a straight line. Understanding the derivative helps in determining the type of function and how it behaves throughout its domain.
Referring to a constant rate of change, a constant derivative suggests that the function changes by a fixed amount every time its input changes by one unit. This is typical of linear functions, where the graph is a straight line. Understanding the derivative helps in determining the type of function and how it behaves throughout its domain.
- The derivative is what tells us if a function is increasing or decreasing.
- If the derivative is positive, the function is increasing.
- If the derivative is negative, the function is decreasing.
- A zero derivative indicates a possible maximum or minimum point (or a flat segment).
Slope
Slope is a concept borrowed from mountains and hills to describe the steepness of a line. In the context of functions, it refers to how fast or slow a function changes as its input changes. It's the "rise over run" — the amount the function goes up or down for a unit increase in the input.
In linear functions, which are our focus here, this slope is always constant and can be directly read from the derivative. For example, a derivative of 2 means every step to the right (increase in \(x\)) causes a step upwards by 2 (increase in \(f(x)\)).
In linear functions, which are our focus here, this slope is always constant and can be directly read from the derivative. For example, a derivative of 2 means every step to the right (increase in \(x\)) causes a step upwards by 2 (increase in \(f(x)\)).
- For linear functions, the slope is consistent for any segment of the line.
- In mathematical terms, the slope for a linear function \(y = mx + b\) is \(m\).
- This consistency makes linear functions easy to predict and understand.
Constant Rate of Change
When we hear "constant rate of change," in mathematics, it's a signal that our function is changing at a steady pace. This concept is clear in linear functions. Here, because the change is constant, every increment in the \(x\)-value results in the same amount of increase or decrease in the \(f(x)\)-value.
In our example, the constant rate of change is represented by the number 2 in the linear function \(f(x) = 2x + 5\). This tells us that as \(x\) increases by 1 unit, \(f(x)\) will increase by 2 units consistently.
In our example, the constant rate of change is represented by the number 2 in the linear function \(f(x) = 2x + 5\). This tells us that as \(x\) increases by 1 unit, \(f(x)\) will increase by 2 units consistently.
- A constant rate of change is a hallmark of linear functions.
- This makes calculations easy and predictable.
- Analyzing constant rates helps in projecting future values and understanding the linear relationship.
Initial Condition
The initial condition of a function is like the starting point of a journey. It tells us where the function begins before any changes occur. In terms of graphs, it represents where the line intersects the y-axis.
In our exercise, we were given the initial condition \(f(0)=5\). This starting point is crucial for determining the specific function we need. It helps find the \(b\) value — an essential part of the line equation \(y = mx + b\).
In our exercise, we were given the initial condition \(f(0)=5\). This starting point is crucial for determining the specific function we need. It helps find the \(b\) value — an essential part of the line equation \(y = mx + b\).
- Initial conditions are vital for defining a function completely.
- They provide a base value from which all changes are measured.
- Without initial conditions, a function can have an infinite number of possibilities based on the same rate of change.