# Derivative of Cosine Squared, cos^2(x) with Proof and Graphs

A Squared Cosine Function is the function cosine x when raised to the power of two. The derivative of cosine squared is equal to minus sine of 2x, -sin(2x). We can find or prove this derivative using the chain rule and the derivatives of the fundamental trigonometric functions.

In this article, we’ll look at how to calculate the composite function cosine squared. We will go over the principles, definition, formula, graph comparison of underived and derived cosine x squared, proof, techniques to derive, and a few examples.

##### CALCULUS

Relevant for

Learning how to find the derivative of cosine squared.

See proof

##### CALCULUS

Relevant for

Learning how to find the derivative of cosine squared.

See proof

## Proof of The Derivative of Cosine Squared Function Using Chain Rule

If you need it, you are encouraged to review the chain rule formula, as a prerequisite of this topic, by visiting this link: Chain Rule of derivatives. Likewise, you may visit this another link for the proof of the derivative of cosine function: Derivative of Cosine, cos(x).

Please take note that

$latex \cos^{2}{(x)} \neq \cos{(x^2)}$

Setting confusion aside, the former is a “whole trigonometric function” raised to the power of two, whereas the latter is a trigonometric function of “a variable raised to the power of two.”

Because it is a composite function, the chain rule formula is used to find the derivative formula of the cosine squared function, provided you have already mastered the chain rule formula and the derivative of cosine function.

Suppose we are asked to get the derivative of

$latex F(x) = \cos^{2}{(x)}$

We can identify the two functions that make up F(x). There is a power function and a trigonometric function in this scenario. Based on our given F(x), they are a function raised to a power of two and a cosine trigonometric function.

For easier representation, we can rewrite our given as

$latex \frac{dy}{dx} = \cos^{2}{(x)}$

$latex \frac{dy}{dx} = (\cos{(x)})^2$

It is evident now that the given power function is the outer function, while the cosine function squared by the given power function is the inner function. We can set the outer function as

$latex f(u) = u^2$

where

$latex u = \cos{(x)}$

Setting the trigonometric cosine function as the inner function of f(u) by denoting it as g(x), we have

$latex f(u) = f(g(x))$

$latex u = g(x)$

$latex g(x) = \cos{(x)}$

Deriving the outer function f(u) using the power rule in terms of u, we have

$latex f(u) = u^2$

$latex f'(u) = 2u$

Deriving the inner function g(x) using the derivative formula of trigonometric function cosine in terms of x, we have

$latex g(x) = \cos{(x)}$

$latex g'(x) = -\sin{(x)}$

Algebraically multiplying the derivative of outer function $latex f'(u)$ by the derivative of inner function $latex g'(x)$, we have

$latex \frac{dy}{dx} = f'(u) \cdot g'(x)$

$latex \frac{dy}{dx} = (2u) \cdot (-\sin{(x)})$

Substituting u into f'(u), we have

$latex \frac{dy}{dx} = (2(\cos{(x)})) \cdot (-\sin{(x)})$

$latex \frac{dy}{dx} = -2\cos{(x)}\sin{(x)}$

$latex \frac{dy}{dx} = -2\sin{(x)}\cos{(x)}$

Applying the double angle identities, we have

$latex \frac{dy}{dx} = -(2\sin{(x)}\cos{(x)})$

$latex \frac{dy}{dx} = -(\sin{(2x)})$

This gets us to the cosine squared x derivative formula.

$latex \frac{d}{dx} \cos^{2}{(x)} = -\sin{(2x)}$

## How to derive a Cosine Squared Function?

As noted previously, cosine squared is a composite function of power and the trigonometric function cosine. Instead of constantly using the chain rule method, we may simply utilize the established derivative formula for a cosine squared function.

### METHOD 1: When the square of a cosine of any angle x is to be derived in terms of the same angle x.

Step 1: Analyze if the cosine squared of an angle is a function of that same angle. For example, if the right-hand side of the equation is $latex \cos^{2}{(x)}$, then check if it is a function of the same angle x or f(x). After this, proceed to Step 2 until you complete the derivation steps.

Note: If $latex \cos^{2}{(x)}$ is a function of a different angle or variable such as f(t) or f(y), it will use implicit differentiation which is out of the scope of this article.

Step 2: Then directly apply the proven derivative formula of the cosine squared function

$latex \frac{dy}{dx} = -\sin{(2x)}$

If nothing is to be simplified anymore, then that would be the final answer.

### METHOD 2: When the given is a cosine squared of any function v instead and to be derived in terms of x.

Step 1: Express the function as $latex G(x) = \cos^{2}{(v)}$, where $latex v$ represents any function other than x.

Step 2: Consider $latex \cos^{2}{(v)}$ as the outside function $latex g(v)$ and $latex v$ as the inner function $latex h(x)$ of the composite function $latex G(x)$. Hence we have

$latex g(v) = \cos{(v)}$

and

$latex h(x) = v$

Step 3: Get the derivative of the outer function $latex g(v)$, which must use the derivative of the cosine squared function, in terms of $latex v$.

$latex \frac{d}{du} \left( \cos^{2}{(v)} \right) = -\sin{(2v)}$

Step 4: Get the derivative of the inner function $latex h(x) = v$. Use the appropriate derivative rule that applies to $latex v$.

Step 5: Apply the basic chain rule formula by algebraically multiplying the derivative of outer function $latex g(v)$ by the derivative of inner function $latex h(x)$

$latex \frac{dy}{dx} = \frac{d}{du} (g(v)) \cdot \frac{d}{dx} (h(x))$

$latex \frac{dy}{dx} = -\sin{(2v)} \cdot \frac{d}{dx} (v)$

Step 6: Substitute $latex v$ into $latex g'(v)$

Step 7: Simplify and apply any function law whenever applicable to finalize the answer.

## Graph of Cosine Squared x VS. The Derivative of Cosine Squared x

Given the function

$latex f(x) = \cos^{2}{(x)}$

its graph shows

And as we know by now, by deriving $latex f(x) = \cos^{2}{(x)}$, we get

$latex f'(x) = -\sin{(2x)}$

which if graphed, shows

Illustrating both graphs in one, we have

Looking at the differences between these functions based on those graphs, you can see that the original function $latex f(x) = \cos^{2}{(x)}$ has a domain of

$latex (-\infty,\infty)$ or all real numbers

and exists within the range of

$latex [0,1]$

whereas the derivative $latex f'(x) = -\sin{(2x)}$ has a domain of

$latex (-\infty,\infty)$ or all real numbers

and exists within the range of

$latex [-1,1]$

## Examples

Here are some examples of deriving a cosine squared function applying either the first or second method.

### EXAMPLE 1

Derive: $latex f(\beta) = \cos^{2}{(\beta)}$

Solution: Analyzing the given cosine squared function, it is only a square of a cosine of a single angle $latex \beta$. Therefore, we can use the first method to derive this problem.

Step 1: Analyze if the square of cosine of $latex \beta$ is a function of $latex \beta$. In this problem, it is. Hence, proceed to step 2.

Step 2: Directly apply the derivative formula of the cosine squared function and derive in terms of $latex \beta$. Since no further simplification is needed, the final answer is:

$latex f'(\beta) = -\sin{(2\beta)}$

### EXAMPLE 2

Derive: $latex G(x) = \cos^{2}{(3x+2)}$

Solution: Analyzing the given cosine squared function, it is a square of a cosine of a polynomial function. Therefore, we can use the second method to derive this problem.

Step 1: Express the cosine squared function as $latex G(x) = \cos^{2}{(v)}$, where $latex v$ represents any function other than x. In this problem,

$latex v = 3x+2$

We will substitute this later as we finalize the derivative of the problem.

Step 2: Consider $latex \cos{(v)}$ as the outside function $latex g(v)$ and $latex v$ as the inner function $latex h(x)$ of the composite function $latex G(x)$. For this problem, we have

$latex g(v) = \cos{(v)}$

and

$latex h(x) = v = 3x+2$

Step 3: Get the derivative of the outer function $latex g(v)$, which must use the derivative of the cosine squared function, in terms of $latex v$.

$latex \frac{d}{du} \left( \cos^{2}{(v)} \right) = -\sin{(2v)}$

Step 4: Get the derivative of the inner function $latex h(x)$ or $latex v$. Since our $latex v$ in this problem is a polynomial function, we will use power rule and sum/difference of derivatives to derive $latex v$.

$latex \frac{d}{dx}(h(x)) = \frac{d}{dx} \left(3x+2 \right)$

$latex \frac{d}{dx}(h(x)) = 3$

Step 5: Apply the basic chain rule formula by algebraically multiplying the derivative of outer function $latex g(v)$ by the derivative of inner function $latex h(x)$

$latex \frac{dy}{dx} = \frac{d}{du} (g(v)) \cdot \frac{d}{dx} (h(x))$

$latex \frac{dy}{dx} = -\sin{(2v)} \cdot 3$

Step 6: Substitute $latex v$ into $latex g'(v)$

$latex \frac{dy}{dx} = -\sin{(2v)} \cdot 3$

$latex \frac{dy}{dx} = -\sin{(2(3x+2))} \cdot 3$

Step 7: Simplify and apply any function law whenever applicable to finalize the answer.

$latex \frac{dy}{dx} = -3\sin{(2(3x+2))}$

And the final answer is:

$latex G'(x) = -3\sin{(2(3x+2))}$

or

$latex G'(x) = -3\sin{(6x+4)}$  