Chapter 3: Problem 48
In each of Exercises \(45-50\) use the Chain Rule repeatedly to determine the derivative with respect to \(x\) of the given expression. $$ \left(1+\cos ^{2}(x)\right)^{3 / 2} $$
Short Answer
Expert verified
The derivative is \(-\frac{3}{2}\sin(2x)(1 + \cos^2(x))^{1/2}\).
Step by step solution
01
Identify the Function Layers
The expression we need to differentiate is \((1 + \cos^2(x))^{3/2}\). Notice that this is a composition of multiple functions: an inner function \(u = 1 + \cos^2(x)\) and an outer function \(v = u^{3/2}\).
02
Differentiate the Outer Function
To differentiate the outer function, \(v = u^{3/2}\), use the power rule: \(\frac{dv}{du} = \frac{3}{2}u^{1/2}\). This gives us the formula for the derivative of the outer layer.
03
Differentiate the Inner Function
For the inner function \(u = 1 + \cos^2(x)\), we apply the chain rule again because it is composed of \(\cos(x)\). The derivative of \(\cos^2(x)\) is \(2\cos(x)(-\sin(x))\) by using the chain rule, giving: \(\frac{du}{dx} = -2\cos(x)\sin(x) = -\sin(2x)\).
04
Apply the Chain Rule
Apply the chain rule to find \(\frac{dv}{dx}\). Use the relation \(\frac{dv}{dx} = \frac{dv}{du} \cdot \frac{du}{dx}\). Substitute the derivatives from the previous steps: \(\frac{dv}{dx} = \frac{3}{2}u^{1/2} \cdot (-\sin(2x))\).
05
Substitute Back the Inner Function
Substitute back the inner function \(u = 1 + \cos^2(x)\) into the expression for \(\frac{dv}{dx}\): \(\frac{dv}{dx} = \frac{3}{2}(1 + \cos^2(x))^{1/2} \cdot (-\sin(2x))\).
06
Simplify the Expression
Rewrite the expression to reflect multiplication: \(\frac{dv}{dx} = -\frac{3}{2}\sin(2x)(1 + \cos^2(x))^{1/2}\). This is the simplified form of the derivative of the given function.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Composite Functions
In calculus, composite functions play a crucial role when differentiating complex expressions. A composite function occurs when one function is inside another. This nested structure requires a specific approach to handle differentiation effectively, known as the Chain Rule.
- Consider a composite function like \( (1 + \cos^2(x))^{3/2} \). Here, we have an inner function \( u = 1 + \cos^2(x) \) and an outer function \( v = u^{3/2} \).
- To differentiate a composite function, we differentiate the outside function, leaving the inside function unchanged, then multiply by the derivative of the inner function.
- This method ensures that all layers of the composite function are appropriately considered in succession.
Power Rule
The Power Rule is a basic yet powerful tool in calculus for differentiating expressions involving exponents. It states that to differentiate \( x^n \), where \( n \) is a constant, the derivative is \( nx^{n-1} \).
- In our example, after identifying \( u = 1 + \cos^2(x) \), we apply the power rule to the outer function \( v = u^{3/2} \).
- The derivative of \( v \) with respect to \( u \) is \( \frac{3}{2}u^{1/2} \).
Trigonometric Derivatives
Trigonometric functions often appear in calculus, and their derivatives are essential tools in differentiating related expressions.
- Sine and cosine, two foundational trigonometric functions, have derivatives: \( \frac{d}{dx}[\sin(x)] = \cos(x) \) and \( \frac{d}{dx}[\cos(x)] = -\sin(x) \).
- When addressing \( \cos^2(x) \), we utilize these derivatives along with the chain rule. The derivative becomes \( 2\cos(x)(-\sin(x)) \) or \(-2\cos(x)\sin(x) \), which simplifies to \( -\sin(2x) \).