iGCSE (2021 Edition)

# 11.17 Differentiating trigonometric functions

Lesson

## The derivatives of $\sin x$sinx and $\cos x$cosx

### Graphical approximation of the derivatives

The sine and cosine functions are periodic, meaning that those functions go through cycles, or repeat.

The period of both the basic sine and cosine functions, $\sin x$sinx and $\cos x$cosx, is $2\pi$2π.

From the graph, it is clear that the gradient of the sine function also varies periodically - from a maximum of $1$1 at $x=0$x=0, through zero at $x=\frac{\pi}{2}$x=π2, to a minimum of $-1$1 at $x=\pi$x=π, back to zero at $x=\frac{3\pi}{2}$x=3π2 and then returning to $1$1 at $x=2\pi$x=2π. This cycle is then repeated.

The following graph shows the sine function and the cosine function.

The graphs strongly suggest that if $f\left(x\right)=\sin x$f(x)=sinx, then the derivative is $f'\left(x\right)=\cos x$f(x)=cosx.

We can undertake a similar process for $f\left(x\right)=\cos x$f(x)=cosx. The graph below shows $f\left(x\right)=\cos x$f(x)=cosx and $g\left(x\right)=-\sin\left(x\right)$g(x)=sin(x). The gradient of $f\left(x\right)=\cos x$f(x)=cosx at $x=0,\pi$x=0,π is equivalent to the function value of $f\left(x\right)=-\sin x$f(x)=sinx at these points. Observation of these graphs suggest that if $f\left(x\right)=\cos x$f(x)=cosx, then $f'\left(x\right)=-\sin\left(x\right)$f(x)=sin(x) .

For $f\left(x\right)=\sin x$f(x)=sinx, the derivative is:

 $f'\left(x\right)$f′(x) $=$= $\cos x$cosx

For $f\left(x\right)=\cos x$f(x)=cosx the derivative is:

 $f'\left(x\right)$f′(x) $=$= $-\sin x$−sinx

These rules for the derivatives of the basic sine and cosine functions can be summarised as follows:

Derivatives of $\sin x$sinx and $\cos x$cosx

For $f\left(x\right)=\sin x$f(x)=sinx, $f'\left(x\right)=\cos x$f(x)=cosx

For $f\left(x\right)=\cos x$f(x)=cosx, $f'\left(x\right)=-\sin x$f(x)=sinx

#### Practice question

##### Question 1

Consider the graphs of $y=\cos x$y=cosx and its derivative $y'=-\sin x$y=sinx below. A number of points have been labelled on the graph of $y'=-\sin x$y=sinx.

1. Which point on the gradient function corresponds to where the graph of $y=\cos x$y=cosx is increasing most rapidly?

$A$A

A

$B$B

B

$C$C

C

$D$D

D

$E$E

E

$A$A

A

$B$B

B

$C$C

C

$D$D

D

$E$E

E
2. Which point on the gradient function corresponds to where the graph of $y=\cos x$y=cosx is decreasing most rapidly?

$A$A

A

$B$B

B

$C$C

C

$D$D

D

$E$E

E

$A$A

A

$B$B

B

$C$C

C

$D$D

D

$E$E

E
3. Which points on the gradient function corresponds to where the graph of $y=\cos x$y=cosx is stationary?

Select all that apply.

$A$A

A

$B$B

B

$C$C

C

$D$D

D

$E$E

E

$A$A

A

$B$B

B

$C$C

C

$D$D

D

$E$E

E

### The derivatives of $\sin\left(f\left(x\right)\right)$sin(f(x)) and $\cos\left(f\left(x\right)\right)$cos(f(x))

We can develop rules for the differentiation of functions in the form $\sin\left(f\left(x\right)\right)$sin(f(x)) and $\cos\left(f\left(x\right)\right)$cos(f(x)) using the chain rule.

Consider functions of the form:

 $y$y $=$= $\sin\left(f\left(x\right)\right)$sin(f(x))

Letting $u=f\left(x\right)$u=f(x), we have:

 $y$y $=$= $\sin u$sinu $\frac{dy}{du}$dydu​ $=$= $\cos u$cosu $\frac{du}{dx}$dudx​ $=$= $f'\left(x\right)$f′(x)

Using the chain rule:

 $\frac{dy}{dx}$dydx​ $=$= $\frac{dy}{du}\times\frac{du}{dx}$dydu​×dudx​ $=$= $\cos u\times u'$cosu×u′ $=$= $f'\left(x\right)\cos\left(f\left(x\right)\right)$f′(x)cos(f(x))

Similarly for functions of the form:

 $y$y $=$= $\cos\left(f\left(x\right)\right)$cos(f(x))

Letting $u=f\left(x\right)$u=f(x), we have:

 $y$y $=$= $\cos u$cosu $\frac{dy}{du}$dydu​ $=$= $-\sin u$−sinu $\frac{du}{dx}$dudx​ $=$= $f'\left(x\right)$f′(x)

Using the chain rule:

 $\frac{dy}{dx}$dydx​ $=$= $\frac{dy}{du}\times\frac{du}{dx}$dydu​×dudx​ $=$= $-\sin u\times u'$−sinu×u′ $=$= $-f'\left(x\right)\sin\left(f\left(x\right)\right)$−f′(x)sin(f(x))

Differentiating $y=\sin\left(f\left(x\right)\right)$y=sin(f(x)) and $y=\cos\left(f\left(x\right)\right)$y=cos(f(x))
 $\frac{d}{dx}\sin\left(f\left(x\right)\right)$ddx​sin(f(x)) $=$= $f'\left(x\right)\cos\left(f\left(x\right)\right)$f′(x)cos(f(x)) $\frac{d}{dx}\cos\left(f\left(x\right)\right)$ddx​cos(f(x)) $=$= $-f'\left(x\right)\sin\left(f\left(x\right)\right)$−f′(x)sin(f(x))

#### Practice questions

##### Question 2

Differentiate $y=8\sin\left(\frac{x}{4}\right)$y=8sin(x4).

1. Write each line of working as an equation.

##### Question 3

Differentiate $y=\cos\left(3x^2\right)$y=cos(3x2).

## The derivative of the tangent function $y=\tan x$y=tanx

The tangent function is defined, in right-angled triangles, to be the ratio of sides $\frac{\text{opposite}}{\text{adjacent}}$oppositeadjacent with respect to one of the non right-angles in the triangle. More generally, we use the unit circle definitions of the trigonometric functions in order to give meaning to functions of any angle. The tangent function is defined in terms of the sine and cosine functions.

$\tan x=\frac{\sin x}{\cos x}$tanx=sinxcosx, for any $x$x not an odd multiple of $\frac{\pi}{2}$π2.

The graph below shows the tangent function between $-\pi$π and $\pi$π.

Observe that the function has asymptotes at $\pm\frac{\pi}{2}$±π2 which are repeated every $\pi$π radians in both directions. These asymptotes correspond to the points where the cosine function in the denominator of the identity $\tan x=\frac{\sin x}{\cos x}$tanx=sinxcosx is equal to zero.

From the graph, it can be seen that the gradient of the tangent function is always positive and it has a minimum value of $1$1 at $x=0\pm n\pi$x=0±nπ. The gradient has no maximum value.

We deduce the tangent function explicitly by differentiating $f(x)=\tan x=\frac{\sin x}{\cos x}$f(x)=tanx=sinxcosx. It is convenient to use the quotient rule for this as follows:

 $f'(x)$f′(x) $=$= $\frac{\cos x\times\cos x-\left(-\sin x\times\sin x\right)}{\cos^2x}$cosx×cosx−(−sinx×sinx)cos2x​ $=$= $\frac{\cos^2x+\sin^2x}{\cos^2x}$cos2x+sin2xcos2x​

Using the Pythagorean trig identity $\sin^2x+\cos^2x=1$sin2x+cos2x=1:

 $f'(x)$f′(x) $=$= $\frac{1}{\cos^2x}$1cos2x​ $=$= $\sec^2x$sec2x

In the diagram below, the graph of $\sec\left(x^2\right)$sec(x2) (in red) has been superimposed on the tangent graph. It confirms the observations made above about the gradient of the tangent function.

It is essential that the argument of the tangent function be expressed in radian measure. This requirement comes from the way in which the derivative of the sine function was obtained.

Derivative of $\tan x$tanx

For $f\left(x\right)=\tan x$f(x)=tanx, the derivative is given by

$f'\left(x\right)=\sec^2x$f(x)=sec2x, where $x$x is in radians

### The derivative of $\tan\left(f\left(x\right)\right)$tan(f(x))

The derivative of $\tan\left(f\left(x\right)\right)$tan(f(x)) can also be obtained using the chain rule as follows:

Consider functions of the form:

 $y$y $=$= $\tan\left(f\left(x\right)\right)$tan(f(x))

Letting $u=f\left(x\right)$u=f(x), we have:

 $y$y $=$= $\tan u$tanu $\frac{dy}{du}$dydu​ $=$= $\sec^2u$sec2u $\frac{du}{dx}$dudx​ $=$= $f'\left(x\right)$f′(x)

Using the chain rule:

 $\frac{dy}{dx}$dydx​ $=$= $\frac{dy}{du}\times\frac{du}{dx}$dydu​×dudx​ $=$= $\sec^2u\times u'$sec2u×u′ $=$= $f'\left(x\right)\sec^2\left(f\left(x\right)\right)$f′(x)sec2(f(x))

Derivative of $\tan\left(f\left(x\right)\right)$tan(f(x))

$\frac{d}{dx}\tan\left(f\left(x\right)\right)=f'(x)\sec^2\left(f\left(x\right)\right)$ddxtan(f(x))=f(x)sec2(f(x)), where $x$x is in radians

#### Worked example

##### Example 1

Differentiate $\tan\left(x^\circ\right)$tan(x°).

Think: First, the argument $x^\circ$x° needs to be converted to radians. To convert a degrees measure to radians, we can multiply it by $\frac{\pi}{180}$π180. This means $x^\circ$x° is equal to $\frac{x\pi}{180}$xπ180 radians.

Do: Now we can look to differentiate $\tan\left(\frac{x\pi}{180}\right)$tan(xπ180).

Using the chain rule, we get $\frac{d}{dx}\tan\left(\frac{x\pi}{180}\right)=\frac{\pi}{180}\sec^2\left(\frac{x\pi}{180}\right)$ddxtan(xπ180)=π180sec2(xπ180).

#### Practice question

##### Question 4

Differentiate $y=-4\tan5x$y=4tan5x.

## Differentiation of harder trigonometric functions

When faced with finding the derivative of a more complicated trigonometric function, we can utilise the product, quotient and other rules to arrive at a solution.

#### Practice questions

##### Question 5

Differentiate $y=\frac{\tan x}{x^2}$y=tanxx2.

##### Question 6

Differentiate $y=\sqrt{\tan6x}$y=tan6x.

##### Question 7

Find the derivative of $x^{\frac{7}{4}}\cos2x$x74cos2x.

### Outcomes

#### 0606C14.3B

Use the derivatives of the standard functions sinx, cos x, tan x, together with constant multiples, sums and composite functions.