How to Differentiate Trigonometric Reciprocals

To calculate the derivatives of reciprocal trigonometric functions (secant, cosecant, and cotangent), you’ll use the basic trigonometric functions (sine, cosine, and tangent) along with the chain rule or quotient rule. Here are the formulas and steps:

Secant:

If secant only had one variable in it ( \(x\) for example) :

1. \( f(x) = \sec(x) = \frac{1}{\cos(x)} \)
2. \( f'(x) = \frac{d}{dx}\left(\frac{1}{\cos(x)}\right) \)
3. Use quotient rule: \( \frac{d}{dx}\left(\frac{1}{u}\right) = -\frac{u’}{u^2} \) where \( u = \cos(x) \)
4. \( f'(x) = -\frac{-\sin(x)}{\cos(x)^2} \)
5. Simplify: \( f'(x) = \tan(x)\sec(x) \)

If secant had a function in it ( \(f(x)\) for example) :

1. \( g(x) = \sec(f(x)) = \frac{1}{\cos(f(x))} \)
2. \( g'(x) = \frac{d}{dx}\left(\frac{1}{\cos(f(x))}\right) \)
3. Use the chain rule and quotient rule: \( \frac{d}{dx}\left(\frac{1}{u}\right) = -\frac{u’}{u^2} \), where \( u = \cos(f(x)) \)
4. \( g'(x) = -\frac{-\sin(f(x)) \cdot f'(x)}{\cos(f(x))^2} \) (Here, \( f'(x) \) is the derivative of \( f(x) )\)
5. Simplify: \( g'(x) = \tan(f(x))\sec(f(x))f'(x) \)

Here is an example:

Let’s find the derivative of the function \( y = \sec(x^2) \). In this case, \( f(x) = x^2 \).

To differentiate \( y = \sec(x^2) \), we will use the formula:
\( \frac{d}{dx} \sec(u) = \sec(u) \tan(u) \frac{du}{dx} \)

Here, \( u = x^2 \), so we need to find \( \frac{du}{dx} \), which is the derivative of \( x^2 \).

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1. First, differentiate \( u = x^2 \) with respect to \( x \):
   \( \frac{du}{dx} = 2x \)
2. Then, apply the formula:
   \( \frac{d}{dx} \sec(x^2) = \sec(x^2) \tan(x^2) \cdot 2x \)

Therefore, the derivative of \( y = \sec(x^2) \) is \( 2x \sec(x^2) \tan(x^2) \).

Cosecant:

Same as previous one, for cosecant of \(x\):

1. \( h(x) = \csc(x) = \frac{1}{\sin(x)} \)
2. \( h'(x) = \frac{d}{dx}\left(\frac{1}{\sin(x)}\right) \)
3. Use quotient rule: \( \frac{d}{dx}\left(\frac{1}{u}\right) = -\frac{u’}{u^2} \), where \( u = \sin(x) \)
4. \( h'(x) = -\frac{\cos(x)}{\sin(x)^2} \)
5. Simplify: \( h'(x) = -\cot(x)\csc(x) \)

and for \(csc(f(x))\):

1. \( h(x) = \csc(f(x)) = \frac{1}{\sin(f(x))} \)
2. \( h'(x) = \frac{d}{dx}\left(\frac{1}{\sin(f(x))}\right) \)
3. Use the chain rule and quotient rule: \( \frac{d}{dx}\left(\frac{1}{u}\right) = -\frac{u’}{u^2} \), where \( u = \sin(f(x)) \)
4. \( h'(x) = -\frac{\cos(f(x)) \cdot f'(x)}{\sin(f(x))^2} \) (Here, \( f'(x) \) is the derivative of \( f(x) )\)
5. Simplify: \( h'(x) = -\cot(f(x))\csc(f(x))f'(x) \)

here is an example:

We’ll use the function \( y = \csc(3x) \) as an example, where \( f(x) = 3x \).

The formula for the derivative of \( \csc(u) \), where \( u \) is a function of \( x \), is given by:
\( \frac{d}{dx} \csc(u) = -\csc(u) \cot(u) \frac{du}{dx} \)

Applying this formula to \( y = \csc(3x) \), we get:

1. First, find \( \frac{du}{dx} \) where \( u = 3x \):
   \( \frac{du}{dx} = 3 \)
2. Then, apply the derivative formula:
   \( \frac{d}{dx} \csc(3x) = -\csc(3x) \cot(3x) \cdot 3 \)

Therefore, the derivative of \( y = \csc(3x) \) is \( -3 \csc(3x) \cot(3x) \).

Cotangent:

To find the derivative of \(cot(x)\):

Sure, here’s the mathematical process for finding the derivative of \(\cot(x)\) in a similar format to the previous ones:

1. \( k(x) = \cot(x) = \frac{\cos(x)}{\sin(x)} \)
2. \( k'(x) = \frac{d}{dx}\left(\frac{\cos(x)}{\sin(x)}\right) \)
3. Use quotient rule: \( \frac{d}{dx}\left(\frac{v}{u}\right) = \frac{v’u – vu’}{u^2} \), where \( v = \cos(x) \) and \( u = \sin(x) \)
4. \( k'(x) = \frac{-\sin(x)\cos(x) – \cos(x)(-\sin(x))}{\sin(x)^2} \)
5. Simplify: \( k'(x) = -\cot(x)^2 – 1 \)
6. Using the Pythagorean identity: \( \cot(x)^2 + 1 = \csc(x)^2 \) Therefore, the expression can also be written as: \( k'(x) = -\csc(x)^2 \)

and now, for the \( cot(f(x)) \):

1. \( k(x) = \cot(f(x)) = \frac{\cos(f(x))}{\sin(f(x))} \)
2. \( k'(x) = \frac{d}{dx}\left(\frac{\cos(f(x))}{\sin(f(x))}\right) \)
3. Use the chain rule and quotient rule: \( \frac{d}{dx}\left(\frac{v}{u}\right) = \frac{v’u – vu’}{u^2} \), where \( v = \cos(f(x)) \) and \( u = \sin(f(x)) \)
4. \( k'(x) = \frac{-\sin(f(x))\cos(f(x))f'(x) – \cos(f(x))(-\sin(f(x))f'(x))}{\sin(f(x))^2} \)
5. Simplify: \( k'(x) = (-\cot(f(x))^2 – 1)f'(x) \)
6. Alternatively, using the identity \( \cot(x)^2 + 1 = \csc(x)^2 ): ( k'(x) = -\csc(f(x))^2f'(x) \)

Here is an example:

Let’s find the derivative of the function \( y = \cot(\sqrt{3x}) \), where \( f(x) = \sqrt{3x} \).

To differentiate \( y = \cot(\sqrt{3x}) \), we use the formula:
\( \frac{d}{dx} \cot(u) = -\csc^2(u) \frac{du}{dx} \)

Here, \( u = \sqrt{3x} \), so we need to find \( \frac{du}{dx} \), the derivative of \( \sqrt{3x} \).

1. First, differentiate \( u = \sqrt{3x} \) with respect to \( x \):
   \( \frac{du}{dx} = \frac{d}{dx} \sqrt{3x} = \frac{d}{dx} (3x)^{1/2} = \frac{1}{2}(3x)^{-1/2} \cdot 3 = \frac{3}{2\sqrt{3x}} \)
2. Then, apply the formula:
   \( \frac{d}{dx} \cot(\sqrt{3x}) = -\csc^2(\sqrt{3x}) \cdot \frac{3}{2\sqrt{3x}} \)

Therefore, the derivative of \( y = \cot(\sqrt{3x}) \) is \( -\frac{3}{2\sqrt{3x}} \csc^2(\sqrt{3x}) \).

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