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# Derivatives of Sin, Cos and Tan

Electromagnetic waves are used to describe a wide variety of phenomena, including radio waves. These waves are intercepted by antennas and the information contained in the wave is processed, giving us means of communication.

An antenna used for telecommunications, pixabay.com

The periodic behavior of waves is often described using trigonometric functions such as sine, cosine, and tangent. The derivatives of trigonometric functions are also required if we were to study the propagation of waves.

## Derivatives of Sin, Cos, and Tan

The derivatives of the sine function, the cosine function, and the tangent function all involve more trigonometric functions.

The derivatives of the sine function, the cosine function, and the tangent function are given as follows:

The derivatives of these trigonometric functions, along with basic differentiation rules, can be used to find the derivatives of the other trigonometric functions: secant, cosecant, and cotangent. Let's first take a look at some examples involving the trigonometric functions sine, cosine, and tangent.

## Derivatives of Sin, Cos, and Tan: Examples

Let's start with the derivative of a function involving the sine function.

Consider the function. We will find its derivative using the derivative of the sine function, the Chain Rule, and the Power Rule.

Let and differentiate using the Chain Rule.

Differentiate the sine function.

Find using the Power Rule.

Substitute back and .

Rearrange the equation.

We will now find the derivative of a function involving the cosine function.

Consider the function. We will find its derivative using the derivative of the cosine function, the Power Rule, and the Chain Rule. Do not forget that the derivative of the cosine function is the negative of the sine function!

Let and differentiate using the chain rule.

Differentiate using the Power Rule.

Find by differentiating the cosine function.

Substitute back and .

Rearrange.

The derivate of a function involving the tangent function is straightforward. Let's take a look at one more example.

Consider the function . We will find its derivative using the derivative of the tangent function, the Chain Rule, and the Power Rule.

Let and differentiate using the chain rule.

Differentiate the tangent function.

Find using the Power Rule.

Substitute back and .

Rearrange.

We have been using the differentiation rules for these trigonometric functions without proving them. Let's now take a look at how to find the derivative of each function.

## Differentiating the Sine Function

The derivative of the sine function can be found by using the definition of the derivative of a function.

We can now use the identity for the sine of the sum of two angles to rewrite the above expression.

This can be rewritten using algebra and the properties of limits.

The value of the involved limits can be found by using The Squeeze Theorem.

and

We find the derivative of the sine function by substituting the above expressions.

For this derivation, we used the values of two limits without proving them. For the sake of completeness, let's dive into their proof!

We will first prove the limit. Consider the unit circle and the triangles in the following diagram.

Diagram showing different areas related to angle h, Heichi Yanajara - StudySmarter Originals

Let be the area of the isosceles triangle, the area of the circular sector, and the area of the right triangle. The area of the triangles can be found by noting that their base is equal to 1, the height of the triangle is equal to and the height of the triangle is equal to.

and

We can find the area with the formula for the area of a circular sector.

Note that contains, which in turn contains. This means that we can set the following inequality:

By substituting the expressions for each area in the above inequality we can write the following:

Next, we divide the whole inequality by :

We can take the reciprocal of each term of the inequality, reversing the inequality signs.

By The Squeeze Theorem, the values of are being squeezed between and 1 as .

Since we can conclude that.

Let's now work on the second limit with some algebra.

We can now use the Pythagorean identity and the product of limits property.

The first limit is equal to 1 as we found previously. The second limit can be evaluated to find that it is equal to 0.

## Differentiating the Cosine Function

The derivative of the cosine function can be found in a similar way.

We can now use the identity for the cosine of the sum of two angles to rewrite the above expression.

Once again, we rewrite this with the help of some algebra and the properties of limits.

Next, we substitute the values of the above limits and find the derivative of the cosine function.

Using the definition of derivative is not the only way to prove the derivative of the cosine function. We can use the derivative of the sine function along with trigonometric identities in our favor!

If we already know the derivative of the sine function we can use the Pythagorean trigonometric identity to find the derivative of the cosine function. Consider the following Pythagorean trigonometric identity:

We can differentiate with respect to both sides of the equation. Since the right-hand side of the equation is equal to a constant, its derivative is equal to 0.

The chain rule can be used on the left-hand side of the equation.

We found previously that the derivative of the sine function is the cosine function, so we will substitute that result in the above equation.

Finally, we divide the equation by and isolate the derivative of .

## Differentiating the Tangent Function

We can also use the definition of a derivative to find the derivative of the tangent function. However, since we already know the derivatives of the sine and the cosine functions we can try using the quotient rule instead. We begin by writing the tangent function as the quotient of the sine function and the cosine function.

Next, we use the quotient rule.

Let's now substitute the derivatives of the sine and the cosine functions.

The numerator can be simplified with the Pythagorean trigonometric identity.

This can be further simplified if we recall that the secant function is the reciprocal of the cosine function.

In this case, using the quotient rule is faster and easier than using the definition of a derivative!

## Differentiating Sin, Cos, and Tan - Key takeaways

• The derivative of the sine function is the cosine function. That is,.
• The derivative of the cosine function is the negative of the sine function. That is,.
• The derivative of the tangent function is the secant function squared. That is,.
• Two important limits are used for proving the derivatives of the sine function and cosine function. These are and .
• The derivative of the tangent function can be found using either the quotient rule or the definition of a derivative.

The derivative of the sine function is the cosine function.

The derivative of the cosine function is the negative sine function.

The derivative of the tangent function is the secant function squared.

The derivatives of the trigonometric functions can be proven using limits and the squeeze theorem.

## Final Derivatives of Sin, Cos and Tan Quiz

Question

Apart from using the definition of a derivative, how can you prove the derivative of the tangent function?

Using the quotient rule and the derivatives of sine and cosine functions.

Show question

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