Problem 6 - Applied Calculus

Question

Differentiate each function. $$ f(t)=\cos \left(t^{3}+t+1\right) $$

Step-by-Step Solution

Answer

The derivative is \( f'(t) = -\sin(t^3 + t + 1) \cdot (3t^2 + 1) \).

Step 1: Identify the overall function and the inner function

The function given is \( f(t) = \cos(t^3 + t + 1) \). We see that this is a composite function where the outer function is cosine \( \cos(u) \), and the inner function is \( u(t) = t^3 + t + 1 \).

Step 2: Derive the outer function

The derivative of \( \cos(u) \) with respect to \( u \) is \( -\sin(u) \). We will use this in the chain rule.

Step 3: Derive the inner function

Differentiate the inner function \( u(t) = t^3 + t + 1 \) with respect to \( t \). This gives: \[\frac{du}{dt} = 3t^2 + 1.\]

Step 4: Apply the chain rule

According to the chain rule, the derivative of the composite function \( f(t) = \cos(u) \) with respect to \( t \) is \[\frac{df}{dt} = \frac{d}{du}(\cos(u)) \cdot \frac{du}{dt} = -\sin(u) \cdot (3t^2 + 1).\]

Step 5: Substitute back the inner function

Replace \( u \) with the expression \( t^3 + t + 1 \) to express the derivative in terms of \( t \):\[\frac{df}{dt} = -\sin(t^3 + t + 1) \cdot (3t^2 + 1).\]

Key Concepts

Chain Rule

In calculus, the **chain rule** is a fundamental technique used for differentiating composite functions. It allows us to find the derivative of a function that is composed of two or more functions. This is especially useful because many real-world problems involve such compositions.

When you have a composite function, say \( f(g(x)) \), the chain rule states that the derivative \( f'(g(x)) \cdot g'(x) \) is obtained by differentiating the outer function \( f \) with respect to the inner function \( g \), and then multiplying the result by the derivative of the inner function \( g \) with respect to \( x \).

• Identify the "outside" function and differentiate it as if the "inside" function's content was just an unknown variable.
• Then, differentiate the "inside" function normally.
• Multiply the two results together to get the complete derivative.

This technique saves time and makes differentiation straightforward when dealing with complex nested functions.

Composite Functions

**Composite functions** involve the application of one function inside another. This nesting is characterized by a function taking the output of one function as its input.

For example, in the problem at hand, the function \( f(t) = \cos(t^3 + t + 1) \) is a composite function. Here, \( \cos(u) \) serves as the outer function, and \( u(t) = t^3 + t + 1 \) acts as the inner function.

• The outer function \( \cos \) modifies the result of the inner polynomial function \( t^3+t+1 \).
• This structure allows for analyzing each component separately, simplifying both understanding and computation.
• Recognizing composite functions is key to applying the chain rule effectively.

Understanding composite functions equips us to spot the sequence of operations within a problem, enabling efficient application of differentiation rules.

Trigonometric Functions

**Trigonometric functions** like sine and cosine are fundamental in calculus, especially when dealing with rates of change in waveforms, cycles, and circular motion. These functions define relationships in right-angle triangles and oscillatory phenomena.

In differentiating trigonometric functions:

• The derivative of \( \cos(x) \) is \( -\sin(x) \).
• Trigonometric derivatives often involve additional components like products or powers when nested within composite functions.

When we mixed trigonometric and composite functions, as seen in \( f(t) = \cos(t^3 + t + 1) \), it's crucial to apply the chain rule to achieve the differentiation. The change in the trigonometric function's "inside" variable impacts the overall function's rate of change, making it essential to link their derivatives correctly.

Trigonometric functions are versatile tools that we can integrate with other functions to model a diverse range of phenomena beyond simple geometric interpretations.