Higher Engineering Mathematics, Sixth Edition

288 Higher Engineering Mathematics

0 1 1.5 2 3

2

4

6

8

10

f(x)

x

A D

C

B f(x) 5 x 2

Figure 27.3

(iii) the gradient of chordAD

=

f( 1. 5 )−f( 1 )

1. 5 − 1

=

2. 25 − 1

0. 5

= 2. 5

(iv) ifEis the point on the curve (1.1,f(1.1)) then

the gradient of chordAE

=

f( 1. 1 )−f( 1 )

1. 1 − 1

=

1. 21 − 1

0. 1

= 2. 1

(v) ifFis the point on the curve (1.01,f(1.01)) then

the gradient of chordAF

=

f( 1. 01 )−f( 1 )

1. 01 − 1

=

1. 0201 − 1

0. 01

= 2. 01

Thus as pointBmoves closer and closer to pointAthe

gradient of thechord approaches nearer and nearer to the

value 2. This is called thelimitingvalueof the gradient

of the chordABand whenBcoincides withAthe chord

becomes the tangent to the curve.

27.3 Differentiation from first principles

In Fig. 27.4,AandBare two points very close together

on a curve,δx(deltax)andδy(deltay) representing

small increments in thexandydirections, respectively.

Gradient of chordAB=

δy

δx

;however,

δy=f(x+δx)−f(x).

Hence

δy

δx

=

f(x+δx)−f(x)

δx

.

0

y

f(x)

f(x 1 x)

y

x

x

A(x, y)

B(x 1 x, y 1 y)

Figure 27.4

Asδxapproaches zero,

δy

δx

approaches a limiting value

and the gradient of the chord approaches the gradient of

the tangent atA.

When determining the gradient of a tangent to a curve

there are two notations used. The gradient of the curve

atAin Fig. 27.4 can either be written as

limit

δx→ 0

δy

δx

or limit

δx→ 0

{

f(x+δx)−f(x)

δx

}

InLeibniz notation,

dy

dx

=limit

δx→ 0

δy

δx

Infunctional notation,

f′(x)=limit

δx→ 0

{

f(x+δx)−f(x)

δx

}

dy

dx

is the same asf′(x)and is called thedifferential

coefficientor thederivative. The process of finding the

differential coefficient is calleddifferentiation.

Problem 1. Differentiate from first principle

f(x)=x^2 and determine the value of the gradient

of the curve atx=2.

To ‘differentiate from first principles’ means ‘to find

f′(x)’ by using the expression

f′(x)=limit

δx→ 0

{

f(x+δx)−f(x)

δx

}

f(x)=x^2