2. Function
2
If π₯ and π¦ are two variables related to one another in such a way that each value of π₯
determines exactly one value of π¦, then we say that π¦ is a function of π₯ and it is simply
written as π¦ = π π₯ , where π₯ is an independent variable and π¦ is a dependent variable. The
value of π¦ or π(π₯) is called functional value.
π¦ = π₯2 is a function
For π₯ = Β±1, Β±2, Β±3, β¦ β¦ β¦
We get, π¦ = 1, 4, 9, β¦ β¦ β¦
π¦2 = π₯ ππ, (π¦ = Β± π₯) is not a function
For π₯ = 1, 4, 9, β¦ β¦ β¦
We get, π¦ = Β±1, Β±2, Β±3, β¦ β¦ β¦
1
1
-1
x y
4
2
-2
9
3
-3
1
1
-1
y
x
4
2
-2
9
3
-3
3. Concept of Derivative
3
Differentiation β Instantaneous rate of change of a function with respect to one of
its variables
β to find the slope of the tangent line to the function at a point.
πππππ =
πβππππ ππ πππ ππ‘πππ
πβππππ ππ π‘πππ
=
Ξπ₯
Ξπ‘
Ξπ‘
Ξπ₯
π‘
π₯
5
20
25
15
0
10
0 2 3 4 7 8
5 6
1
Time
Position
9 π‘
π₯
5
20
25
15
0
10
0 2 3 4 7 8
5 6
1
Time
Position
9
13. Example 6: Find the derivative of π π = cos π using first principle
13
Let π¦ = π π₯ = cos π₯
π π₯ = cos π₯
π π₯ + β = cos π₯ + β
= lim
ββ0
cos π₯ + β β cos π₯
β
From First Principle,
ππ¦
ππ₯
= πβ²(π₯) = lim
ββ0
π π₯ + β β π(π₯)
β
= lim
ββ0
β2 sin
2π₯ + β
2
sin
β
2
β
= β lim
ββ0
sin
2π₯ + β
2
Γ lim
β
2
β0
sin
β
2
β
2
= β sin
2π₯ + 0
2
Γ 1
β΄
π π
π π
=
π
π π
cos π = β π¬π’π§ π
cos π β cos π = β2 sin
π + π
2
sin
π β π
2
lim
ββ0
sin β
β
= lim
ββ0
β
sin β
= 1
β΅ β β 0 βΉ
β
2
β 0????
β β 0
βΉ
1
2
Γ β β
1
2
Γ 0
βΉ
β
2
β 0
14. Example 7: Find the derivative of π π = π¬π’π§ ππ using first principle
14
Let π¦ = π π₯ = sin ππ₯
π π₯ = sin ππ₯
π π₯ + β = sin π π₯ + β
= sin ππ₯ + πβ
= lim
ββ0
sin ππ₯ + πβ β sin ππ₯
β
From First Principle,
ππ¦
ππ₯
= πβ²(π₯)
= lim
ββ0
π π₯ + β β π(π₯)
β
= lim
ββ0
2 cos
2ππ₯ + πβ
2
sin
πβ
2
β
= lim
ββ0
cos
2ππ₯ + πβ
2
Γ lim
ββ0
sin
πβ
2
β
2
= cos
2ππ₯ + π. 0
2
Γ lim
ββ0
π sin
πβ
2
πβ
2
ππ¦
ππ₯
= (cos ππ₯) Γ π lim
πβ
2 β0
sin
πβ
2
πβ
2
sin π β sin π = 2 πππ
π + π
2
π ππ
π β π
2
β΄
π π
π π
=
π
π π
π¬π’π§ ππ = π ππ¨π¬ ππ
= (cos ππ₯) Γ π 1
β΅ β β 0 βΉ
πβ
2
β 0
lim
ββ0
sin β
β
= lim
ββ0
β
sin β
= 1
15. Example 8: Find the derivative of π π = πππ π using first principle
15
Let π¦ = π π₯ = tan π₯
π π₯ = tan π₯
π π₯ + β = tan π₯ + β
= lim
ββ0
tan π₯ + β β tan π₯
β
From First Principle,
ππ¦
ππ₯
= πβ²(π₯)
= lim
ββ0
π π₯ + β β π(π₯)
β
= lim
ββ0
sin π₯ + β
cos π₯ + β
β
sin π₯
cos π₯
β
= lim
ββ0
sin π₯ + β cos π₯ β sin π₯ cos π₯ + β
cos π₯ + β cos π₯
β
= lim
ββ0
sin π₯ + β cos π₯ β cos π₯ + β sin π₯
β cos π₯ + β cos π₯
ππ¦
ππ₯
= lim
ββ0
sin β
β cos π₯ + β cos π₯
sin π cos π β cos π sin π = sin π β π
β΄
π π
π π
=
π
π π
πππ§ π = π¬πππ π
= lim
ββ0
sin β
β
Γ lim
ββ0
1
cos π₯ + β cos π₯
= 1 Γ
1
cos π₯ + 0 cos π₯
=
1
cos2 π₯
= sec2 π₯
= lim
ββ0
sin π₯ + β β π₯
β cos π₯ + β cos π₯
19. Exercise
19
Find the derivative of the following functions using first principle:
1. f x = ax
2. f x = cos ax
3. f x = sinβ1 x
4. f x = cosβ1
x
5. f x = sin x2