Differential calculus
G
27
Methods of differentiation
27.1 The gradient of a curve
If a tangent is drawn at a point P on a curve, then the
gradient of this tangent is said to be thegradient of
the curveatP. In Fig. 27.1, the gradient of the curve
atPis equal to the gradient of the tangentPQ.
Figure 27.1
For the curve shown in Fig. 27.2, let the points
A and B have co-ordinates (x 1 ,y 1 ) and (x 2 ,y 2 ),
respectively. In functional notation,y 1 =f(x 1 ) and
y 2 =f(x 2 ) as shown.
Figure 27.2
The gradient of the chordAB=BC
AC=BD−CD
ED=f(x 2 )−f(x 1 )
(x 2 −x 1 )For the curvef(x)=x^2 shown in Fig. 27.3.Figure 27.3(i) the gradient of chordAB=f(3)−f(1)
3 − 1=9 − 1
2= 4(ii) the gradient of chordAC=f(2)−f(1)
2 − 1=4 − 1
1= 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