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IB DP Maths AA SL Study Notes

5.1.1 Power Rule

Formula

The Power Rule is articulated as follows: If you possess a function f(x) = xr, the derivative of this function is:

f'(x) = r * x(r-1)

This rule offers a streamlined process for differentiation, especially when dealing with polynomial functions. For more advanced differentiation techniques, see the Product and Quotient Rules.

Applications

The Power Rule is not just a theoretical concept; it finds extensive application in various mathematical and scientific domains. Its utility spans from addressing basic differentiation problems to handling intricate real-world situations. Let's explore some of its applications in depth:

1. Differentiating Polynomial Functions

Polynomial functions are essentially combinations of terms that have non-negative integer exponents. The Power Rule facilitates the differentiation of each of these terms individually.

Example 1:

Consider the function f(x) = 3x4 - 5x3 + 2x2 - 7x + 1. Let's differentiate this function.

Using the Power Rule:

  • The derivative of the term 3x4 is 12x3.
  • The derivative of the term -5x3 is -15x2.
  • The derivative of the term 2x2 is 4x.
  • The derivative of the term -7x is -7.
  • The constant term 1 has a derivative of zero.

Combining these derivatives, we get:

f'(x) = 12x3 - 15x2 + 4x - 7

For functions involving more complex compositions, the Chain Rule is also a valuable tool.

2. Determining the Slope of Curves

The derivative of a function at a specific point provides the slope of the tangent to the curve at that point. The Power Rule is instrumental in ascertaining this slope.

Example 2:

Let's find the slope of the curve y = x3 at the point x = 2.

Using the Power Rule, the derivative y' is:

y' = 3x2

Evaluating this at x = 2:

y'(2) = 3(22) = 12

This implies that the slope of the curve at x = 2 is 12. For further study on this topic, refer to the Equation of a Tangent Line.

3. Analysing the Behaviour of Functions

The Power Rule is pivotal in discerning the increasing or decreasing nature of functions. If the derivative is positive, it indicates that the function is increasing. Conversely, a negative derivative suggests that the function is decreasing.

Example 3:

Let's analyse the behaviour of the function g(x) = -x5 + 4x3 within the interval [-2, 2].

Differentiating using the Power Rule, we get:

g'(x) = -5x4 + 12x2

To determine where the function is increasing or decreasing, equate g'(x) to zero and solve for x. This will yield the critical points. By testing the intervals between these critical points, one can deduce the behaviour of the function. To understand more about critical points, visit Finding Local Extrema.

4. Real-world Applications

The Power Rule isn't confined to textbooks; it has tangible real-world applications:

  • Physics: In kinematics, the Power Rule can be used to determine velocity from a given position function or acceleration from a velocity function.
  • Economics: Economists use the Power Rule to find the marginal cost or marginal revenue when given a cost or revenue function, respectively.
  • Biology: In population dynamics, the Power Rule can help in understanding the rate of growth or decay of populations.
  • Engineering: Engineers often use the Power Rule to determine rates of change in various scenarios, such as the rate of cooling or heating in thermodynamics. For more on related applications, see Position and Velocity.

5. Further Exploration

While the Power Rule provides a direct method to differentiate power functions, it's essential to understand its limitations and the conditions under which it can be applied. For instance, the function must be differentiable at the point of interest. Moreover, when dealing with functions that aren't purely power functions, one might need to employ other differentiation techniques in conjunction with the Power Rule.

Practice Problems

To solidify understanding, let's delve into some practice problems:

  • Differentiate the function h(x) = 4x7 - 3x6 + x4 - 2x + 5.
  • Find the slope of the curve y = 2x5 - x3 + 3x2 - 4 at x = 1.
  • Determine the intervals where the function j(x) = x4 - 4x3 + 6x2 is increasing or decreasing.

Final Thoughts

The Power Rule is a cornerstone in the world of differentiation. By mastering this rule, students can confidently tackle a myriad of problems in their IB Mathematics coursework. Always ensure to verify the applicability of the rule for the given function and remember that the function should be differentiable at the point in question.

FAQ

Yes, the Power Rule can be applied to functions with negative exponents. In fact, the rule is valid for any real number exponent, whether it's positive, negative, or even a fraction. When differentiating a function with a negative exponent using the Power Rule, the process remains the same. For instance, if you have a function f(x) = x(-3), its derivative using the Power Rule would be f'(x) = -3x(-4). The negative exponent indicates that the function is a reciprocal, and its derivative can be found just as efficiently using the Power Rule.

While the Power Rule is incredibly versatile, it does have some limitations. The primary one is that the function must be differentiable at the point of interest. The Power Rule is specifically designed for functions of the form f(x) = xr, where r is a real number. It cannot be directly applied to functions that aren't in this form without some modification or additional rules. For instance, functions involving trigonometric terms, exponential terms, or logarithmic terms require their own differentiation rules or a combination of rules.

The Power Rule has a direct implication when it comes to the differentiation of constant terms. A constant term can be thought of as a power function where the exponent is 0. For instance, the constant term 5 can be represented as 5x0, since any non-zero number raised to the power of 0 is 1. When differentiating using the Power Rule, the exponent becomes a coefficient, and the original exponent is reduced by one. However, for a constant term (x0), the resulting coefficient is 0, making the derivative of any constant term always zero.

The Power Rule is one of the fundamental differentiation techniques, especially for polynomial functions. It offers a direct and straightforward method for differentiating power functions. However, in calculus, there are various functions and scenarios that the Power Rule alone cannot address. For such cases, other differentiation techniques, like the Product Rule, Quotient Rule, and Chain Rule, come into play. Each of these rules addresses specific types of functions or situations. While the Power Rule is foundational, a comprehensive understanding of calculus requires familiarity with multiple differentiation techniques and the ability to apply them as needed.

The Power Rule is aptly named because it provides a method to differentiate functions that are raised to a power. Specifically, it deals with functions of the form f(x) = xr, where r is a real number. The rule simplifies the process of finding the derivative by focusing on the power (or exponent) of the function. By applying the Power Rule, the exponent becomes a coefficient, and the original exponent is reduced by one. This systematic approach to differentiation, centred around the manipulation of powers, is the reason behind its name.

Practice Questions

Differentiate the function f(x) = 7x^5 - 4x^3 + 3x^2 - 9x + 6 using the Power Rule.

To differentiate the function, we'll apply the Power Rule to each term:

  • The derivative of 7x5 is 35x4.
  • The derivative of -4x3 is -12x2.
  • The derivative of 3x2 is 6x.
  • The derivative of -9x is -9.
  • The constant term 6 has a derivative of zero.

Combining these results, the derivative f'(x) is:

f'(x) = 35x4 - 12x2 + 6x - 9.

Given the function g(x) = 2x^4 - 5x^3 + x - 3, find the slope of the tangent to the curve at x = 2.

First, we need to differentiate the function using the Power Rule:

  • The derivative of 2x4 is 8x3.
  • The derivative of -5x3 is -15x2.
  • The derivative of x is 1.
  • The constant term -3 has a derivative of zero.

Combining these results, the derivative g'(x) is:

g'(x) = 8x3 - 15x2 + 1.

To find the slope of the tangent at x = 2, substitute x = 2 into the derivative:

g'(2) = 8(23) - 15(22) + 1 = 64 - 60 + 1 = 5.

Thus, the slope of the tangent to the curve at x = 2 is 5.

Dr Rahil Sachak-Patwa avatar
Written by: Dr Rahil Sachak-Patwa
LinkedIn
Oxford University - PhD Mathematics

Rahil spent ten years working as private tutor, teaching students for GCSEs, A-Levels, and university admissions. During his PhD he published papers on modelling infectious disease epidemics and was a tutor to undergraduate and masters students for mathematics courses.

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