Setting the Derivative Less Than Zero

Oct 13, 2025 by wbrum.org 37 views

Introduction

In calculus, the concept of increasing and decreasing functions is crucial in understanding the behavior of functions. A function is said to be decreasing on an interval if its derivative is negative on that interval. In this article, we will determine the interval(s) for which the given function is decreasing.

The Function

The given function is:

f(x) = 2x^3 - 5x^2 + 4x - 7

Finding the Derivative

To determine the intervals on which the function is decreasing, we need to find the derivative of the function. The derivative of f(x) is given by:

f'(x) = 6x^2 - 10x + 4

Setting the Derivative Less Than Zero

To find the intervals on which the function is decreasing, we need to set the derivative less than zero and solve for x:

6x^2 - 10x + 4 < 0

Solving the Quadratic Inequality

To solve the quadratic inequality, we can use the quadratic formula:

x = (-b ± √(b^2 - 4ac)) / 2a

In this case, a = 6, b = -10, and c = 4. Plugging these values into the quadratic formula, we get:

x = (10 ± √((-10)^2 - 4(6)(4))) / 2(6) x = (10 ± √(100 - 96)) / 12 x = (10 ± √4) / 12 x = (10 ± 2) / 12

Solving for x

Now, we need to solve for x:

x = (10 + 2) / 12 or x = (10 - 2) / 12 x = 12 / 12 or x = 8 / 12 x = 1 or x = 2/3

Writing the Solution in Interval Notation

The solution to the quadratic inequality is:

(2/3, 1)

Conclusion

In this article, we determined the interval(s) for which the given function is decreasing. The function f(x) = 2x^3 - 5x^2 + 4x - 7 is decreasing on the interval (2/3, 1).

Graph of the Function

The graph of the function f(x) = 2x^3 - 5x^2 + 4x - 7 is shown below:

y 87 6 5 4 3- 2 1 -8-7-6-5-4-3-2-10 -5 + 2 3 4 5 6 7 I

Discussion

The graph of the function shows that the function is decreasing on the interval (2/3, 1). This means that for any value of x in this interval, the function is decreasing.

Key Takeaways

The function f(x) = 2x^3 - 5x^2 + 4x - 7 is decreasing on the interval (2/3, 1).
The derivative of the function is f'(x) = 6x^2 - 10x + 4.
To determine the intervals on which the function is decreasing, we need to set the derivative less than zero and solve for x.

References

[1] Calculus, 3rd edition, Michael Spivak
[2] Calculus, 2nd edition, James Stewart
Determine the Interval(s) for Which the Function is Decreasing: Q&A ====================================================================

Introduction

In our previous article, we determined the interval(s) for which the function f(x) = 2x^3 - 5x^2 + 4x - 7 is decreasing. In this article, we will answer some frequently asked questions related to this topic.

Q: What is the significance of determining the intervals on which a function is decreasing?

A: Determining the intervals on which a function is decreasing is crucial in understanding the behavior of the function. It helps us to identify the points at which the function changes from increasing to decreasing or vice versa.

Q: How do we determine the intervals on which a function is decreasing?

A: To determine the intervals on which a function is decreasing, we need to find the derivative of the function and set it less than zero. We then solve for x to find the intervals on which the function is decreasing.

Q: What is the difference between a function being increasing and decreasing?

A: A function is said to be increasing on an interval if its derivative is positive on that interval. On the other hand, a function is said to be decreasing on an interval if its derivative is negative on that interval.

Q: Can a function be both increasing and decreasing on the same interval?

A: No, a function cannot be both increasing and decreasing on the same interval. If a function is increasing on an interval, it means that its derivative is positive on that interval. Similarly, if a function is decreasing on an interval, it means that its derivative is negative on that interval.

Q: How do we graph a function that is decreasing on an interval?

A: To graph a function that is decreasing on an interval, we need to draw the graph of the function and shade the interval on which the function is decreasing.

Q: Can a function be decreasing on multiple intervals?

A: Yes, a function can be decreasing on multiple intervals. For example, the function f(x) = 2x^3 - 5x^2 + 4x - 7 is decreasing on the intervals (2/3, 1) and (-1, 0).

Q: How do we determine the intervals on which a function is increasing?

A: To determine the intervals on which a function is increasing, we need to find the derivative of the function and set it greater than zero. We then solve for x to find the intervals on which the function is increasing.

Q: What is the relationship between the derivative of a function and its graph?

A: The derivative of a function represents the rate of change of the function with respect to x. The graph of the derivative of a function is called the derivative graph. The derivative graph helps us to understand the behavior of the function.

Q: Can a function have a derivative that is zero on an interval?

A: Yes, a function can have a derivative that is zero on an interval. This means that the function is neither increasing nor decreasing on that interval.

Conclusion

In this article, we answered some frequently asked questions related to determining the intervals on which a function is decreasing. We hope that this article has provided you with a better understanding of this topic.

Key Takeaways

Determining the intervals on which a function is decreasing is crucial in understanding the behavior of the function.
To determine the intervals on which a function is decreasing, we need to find the derivative of the function and set it less than zero.
A function can be decreasing on multiple intervals.
The derivative of a function represents the rate of change of the function with respect to x.

References

[1] Calculus, 3rd edition, Michael Spivak
[2] Calculus, 2nd edition, James Stewart