Grade 11

Grade 11CalculusDifferentiation


Quotient Rule


The quotient rule is a method used in calculus to find the derivative of a function that is the quotient (or division) of two other functions. This rule is necessary when dealing with functions of the form f(x) = g(x) / h(x), where g(x) and h(x) are both functions. The challenge is that these functions can behave quite differently on their own, making calculating their derivatives a little more complicated.

What is the quotient rule?

Mathematically, the quotient rule states that if you have a function given by f(x) = g(x) / h(x), then the derivative f'(x) is given by:

(f(x))' = (g'(x) * h(x) - g(x) * h'(x)) / (h(x)^2)

In simple terms, to find the derivative of a quotient:

  • Differentiate the numerator to get g'(x).
  • Multiply this derivative by the denominator h(x).
  • Differentiate the denominator to get h'(x).
  • Multiply this derivative by the numerator g(x).
  • Subtract the second product from the first product.
  • Finally, divide it by the square of the denominator h(x)^2.

Breaking the quotient rule

Let us analyse the components of the Quotient Rule:

  • g'(x) * h(x): This is the derivative of the numerator and the original denominator.
  • g(x) * h'(x): This is the value of the original numerator multiplied by the derivative of the denominator.
  • h(x)^2: This is the square of the original denominator.

In short, the rule is making sure that we take into account how both functions change, which affects the overall rate of change of the quotient.

Visual example 1: Applying the quotient rule

f(x) = x^2 / (3x + 2) (f(x))' = ((2x) * (3x + 2) - (x^2) * 3) / (3x + 2)^2

Consider a function f(x) = x^2 / (3x + 2) To find the derivative f'(x):

  1. The derivative of the fraction x^2 is 2x.
  2. The derivative of the denominator 3x + 2 is 3.
  3. Apply the quotient rule:
  4. (f(x))' = ((2x) * (3x + 2) - (x^2) * 3) / (3x + 2)^2
        

By simplifying this expression, we get a new function that shows how the original quotient changes.

Lesson example 2: Applying the quotient rule

Let's look at a text example including steps:

Function: f(x) = (5x - 4) / (x + 1)

We need to find the derivative of f'(x):

  1. Identify g(x) = 5x - 4 and h(x) = x + 1.
  2. Differentiate g(x) to get g'(x) = 5.
  3. Differentiate h(x) to get h'(x) = 1.
  4. Apply the quotient rule:
  5. (f(x))' = ((5) * (x + 1) - (5x - 4) * 1) / (x + 1)^2
        
  6. Simplify the expression to find f'(x):
  7. (f(x))' = (5x + 5 - 5x + 4) / (x + 1)^2
    (f(x))' = (9) / (x + 1)^2
        

Visual example 2: Applying the quotient rule

f(x) = sin(x) / x (f(x))' = ((cos(x) * x) - (sin(x) * 1)) / x^2

Consider a function f(x) = sin(x) / x To find the derivative f'(x):

  1. The derivative of the fraction sin(x) is cos(x).
  2. The derivative of every x is 1.
  3. Applying the quotient rule:
  4. (f(x))' = ((cos(x) * x) - (sin(x) * 1)) / x^2
        

The resulting expression represents the rate of change of the original function.

Practice problems

Practice is key to mastering the quotient rule. Try these problems yourself:

  1. Differentiate: f(x) = (3x^3 - x) / (2x^2 + 1)
  2. Find f'(x): f(x) = (7x + 5) / (4x^3 - x)
  3. Calculate the derivative: f(x) = e^x / ln(x)

Conclusion

The quotient rule is a powerful tool in differentiation, enabling you to handle complex functions created by division. Understanding and applying this rule requires careful attention to the order of operations and the relationship between the numerator and denominator. With practice, it becomes an automatic part of solving calculus problems involving quotients.


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