Differentiation of the sine and cosine functions from first ...
Differentiation of
the sine and cosine
functions from
first principles
mc-TY-sincos-2009-1
In order to master the techniques explained here it is vital that you undertake plenty of practice
exercises so that they become second nature.
After reading this text, and/or viewing the video tutorial on this topic, you should be able to:
? differentiate the function sin x from first principles
? differentiate the function cos x from first principles
Contents
1. Introduction
2
2. The derivative of f (x) = sin x
3
3. The derivative of f (x) = cos x
4
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1. Introduction
In this unit we look at how to differentiate the functions f (x) = sin x and f (x) = cos x from first
principles. We need to remind ourselves of some familiar results.
The derivative of f (x).
The definition of the derivative of a function y = f (x) is
f (x + x) ? f (x)
dy
= lim
dx x0
x
Two trigonometric identities.
We will make use of the trigonometric identities
C +D
C?D
sin
2
2
C ?D
C +D
sin
cos C ? cos D = ?2 sin
2
2
sin C ? sin D = 2 cos
The limit of the function
sin
.
As (measured in radians) approaches zero, the function
sin
tends to 1. We write this as
sin
=1
ȡ0
lim
This result can be justified by choosing values of closer and closer to zero and examining the
sin
behaviour of
.
sin
Table 1 shows values of and
as becomes smaller.
1
0.1
0.01
sin
0.84147
0.09983
0.00999
Table 1: The value of
sin
sin
0.84147
0.99833
0.99983
as tends to zero is 1.
You should verify these results with your calculator to appreciate that the value of
proaches 1 as tends to zero.
We now use these results in order to differentiate f (x) = sin x from first principles.
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sin
ap
2. Differentiating f (x) = sin x
Here f (x) = sin x so that f (x + x) = sin(x + x).
So
f (x + x) ? f (x) = sin(x + x) ? sin x
The right hand side is the difference of two sine terms. We use the first trigonometric identity
(above) to write this in an alternative form.
sin(x + x) ? sin x = 2 cos
= 2 cos
x + x + x
x
sin
2
2
2x + x
x
sin
2
2
= 2 cos(x +
x
x
) sin
2
2
Then, using the definition of the derivative
dy
=
dx
f (x + x) ? f (x)
x0
x
lim
2 cos(x + x
) sin x
2
2
=
x
The factor of 2 can be moved into the denominator as follows, in order to write this in an
alternative form:
cos(x + x
) sin x
dy
2
2
=
dx
x/2
x sin x
2
= cos x +
x
2
2
We now let x tend to zero. Consider the term
=
sin x
2
x
2
and use the result that lim
ȡ0
sin
= 1 with
x
. We see that
2
lim
x0
sin x
2
x
2
=1
Further,
x
lim cos x +
x0
2
= cos x
So finally,
dy
= cos x
dx
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3. The derivative of f (x) = cos x.
Here f (x) = cos x so that f (x + x) = cos(x + x).
So
f (x + x) ? f (x) = cos(x + x) ? cos x
The right hand side is the difference of two cosine terms. This time we use the trigonometric
identity
C +D
C ?D
cos C ? cos D = ?2 sin
sin
2
2
to write this in an alternative form.
cos(x + x) ? cos x = ?2 sin
?2 sin
x + x + x
x
sin
2
2
2x + x
x
sin
2
2
= ?2 sin(x +
x
x
) sin
2
2
Then, using the definition of the derivative
dy
=
dx
f (x + x) ? f (x)
x0
x
lim
?2 sin(x + x
) sin x
2
2
=
x
The factor of 2 can be moved as before, in order to write this in an alternative form:
) sin x
sin(x + x
dy
2
2
= ?
dx
x/2
x sin x
2
= ? sin x +
x
2
2
We now want to let x tend to zero. As before
lim
x0
Further,
sin x
2
x
2
x
lim ? sin x +
x0
2
=1
= ? sin x
So finally,
dy
= ? sin x
dx
So, we have used differentiation from first principles to find the derivatives of the functions sin x
and cos x.
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