2. Packet #4 Definition of the Derivative
Math 180
Packet #4
Definition of the Derivative

3. Ex 1. Suppose you drop a calculus book from the top of building 61
Ex 1. Suppose you drop a calculus book from the top of building 61. And suppose you figure out that the distance fallen is 𝑦=16 𝑡 2 (𝑦 is in feet, and 𝑡 is in seconds). What is the average speed of the book between 𝑡=1 sec and 𝑡=2 sec? What is the average speed of the book between 𝑡=1 sec and 𝑡=1.1 sec?

4. Ex 1. Suppose you drop a calculus book from the top of building 61
Ex 1. Suppose you drop a calculus book from the top of building 61. And suppose you figure out that the distance fallen is 𝑦=16 𝑡 2 (𝑦 is in feet, and 𝑡 is in seconds). What is the average speed of the book between 𝑡=1 sec and 𝑡=2 sec? What is the average speed of the book between 𝑡=1 sec and 𝑡=1.1 sec?

5. Ex 1 (continued). What is the speed of the book at 𝑡=1 second
Ex 1 (continued). What is the speed of the book at 𝑡=1 second? (This is called the instantaneous speed at 𝑡=1.)
Length of
time interval
Average speed
over interval
Instantaneous speed:
_____________

6. Ex 1 (continued). What is the speed of the book at 𝑡=1 second
Ex 1 (continued). What is the speed of the book at 𝑡=1 second? (This is called the instantaneous speed at 𝑡=1.)
Length of
time interval
Average speed
over interval
Instantaneous speed:
_____________

7. Ex 1 (continued). What is the speed of the book at 𝑡=1 second
Ex 1 (continued). What is the speed of the book at 𝑡=1 second? (This is called the instantaneous speed at 𝑡=1.)
Length of
time interval
Average speed
over interval
Instantaneous speed:
_____________

8. Ex 1 (continued). What is the speed of the book at 𝑡=1 second
Ex 1 (continued). What is the speed of the book at 𝑡=1 second? (This is called the instantaneous speed at 𝑡=1.)
Length of
time interval
Average speed
over interval
Instantaneous speed:
_____________

9. Ex 1 (continued). What is the speed of the book at 𝑡=1 second
Ex 1 (continued). What is the speed of the book at 𝑡=1 second? (This is called the instantaneous speed at 𝑡=1.)
Length of
time interval
Average speed
over interval
Instantaneous speed:
_____________

10. Ex 1 (continued). We can graph the distance the book has traveled with respect to time.

11. Ex 1 (continued). We can graph the distance the book has traveled with respect to time.
Graphically, an average speed is the ______ of a ___________.

12. Ex 1 (continued). We can graph the distance the book has traveled with respect to time.
Graphically, an average speed is the ______ of a ___________.

13. Ex 1 (continued). We can graph the distance the book has traveled with respect to time.
Graphically, an average speed is the ______ of a ___________.

14. Ex 1 (continued). We can graph the distance the book has traveled with respect to time.
Graphically, an average speed is the ______ of a ___________.
slope

15. Ex 1 (continued). We can graph the distance the book has traveled with respect to time.
Graphically, an average speed is the ______ of a ___________.
slope
secant line

16. Ex 1 (continued). We can graph the distance the book has traveled with respect to time.
Instantaneous speed is the ______ of a ___________.

17. Ex 1 (continued). We can graph the distance the book has traveled with respect to time.
Instantaneous speed is the ______ of a ___________.
slope

18. Ex 1 (continued). We can graph the distance the book has traveled with respect to time.
Instantaneous speed is the ______ of a ___________.
slope
tangent line

19. In general, the _____________________ of 𝑦=𝑓(𝑥) with respect to 𝑥 over the interval 𝑎,𝑏 is: Δ𝑦 Δ𝑥 = 𝑓 𝑏 −𝑓 𝑎 𝑏−𝑎 = 𝑓 𝑎+ℎ −𝑓 𝑎 ℎ

20. average rate of change
In general, the _____________________ of 𝑦=𝑓(𝑥) with respect to 𝑥 over the interval 𝑎,𝑏 is: Δ𝑦 Δ𝑥 = 𝑓 𝑏 −𝑓 𝑎 𝑏−𝑎 = 𝑓 𝑎+ℎ −𝑓 𝑎 ℎ

21. average rate of change
In general, the _____________________ of 𝑦=𝑓(𝑥) with respect to 𝑥 over the interval 𝑎,𝑏 is: Δ𝑦 Δ𝑥 = 𝑓 𝑏 −𝑓 𝑎 𝑏−𝑎 = 𝑓 𝑎+ℎ −𝑓 𝑎 ℎ

22. average rate of change
In general, the _____________________ of 𝑦=𝑓(𝑥) with respect to 𝑥 over the interval 𝑎,𝑏 is: Δ𝑦 Δ𝑥 = 𝑓 𝑏 −𝑓 𝑎 𝑏−𝑎 = 𝑓 𝑎+ℎ −𝑓 𝑎 ℎ

23. average rate of change
In general, the _____________________ of 𝑦=𝑓(𝑥) with respect to 𝑥 over the interval 𝑎,𝑏 is: Δ𝑦 Δ𝑥 = 𝑓 𝑏 −𝑓 𝑎 𝑏−𝑎 = 𝑓 𝑎+ℎ −𝑓 𝑎 ℎ

24. average rate of change
In general, the _____________________ of 𝑦=𝑓(𝑥) with respect to 𝑥 over the interval 𝑎,𝑏 is: Δ𝑦 Δ𝑥 = 𝑓 𝑏 −𝑓 𝑎 𝑏−𝑎 = 𝑓 𝑎+ℎ −𝑓 𝑎 ℎ

25. average rate of change
In general, the _____________________ of 𝑦=𝑓(𝑥) with respect to 𝑥 over the interval 𝑎,𝑏 is: Δ𝑦 Δ𝑥 = 𝑓 𝑏 −𝑓 𝑎 𝑏−𝑎 = 𝑓 𝑎+ℎ −𝑓 𝑎 ℎ

26. average rate of change
In general, the _____________________ of 𝑦=𝑓(𝑥) with respect to 𝑥 over the interval 𝑎,𝑏 is: Δ𝑦 Δ𝑥 = 𝑓 𝑏 −𝑓 𝑎 𝑏−𝑎 = 𝑓 𝑎+ℎ −𝑓 𝑎 ℎ

27. average rate of change
In general, the _____________________ of 𝑦=𝑓(𝑥) with respect to 𝑥 over the interval 𝑎,𝑏 is: Δ𝑦 Δ𝑥 = 𝑓 𝑏 −𝑓 𝑎 𝑏−𝑎 = 𝑓 𝑎+ℎ −𝑓 𝑎 ℎ

28. As ℎ (our interval width) approaches 0, the slopes of the secant lines approach the slope of the tangent line. With limits, we write the tangent slope like this: 𝐥𝐢𝐦 𝒉→𝟎 𝒇 𝒂+𝒉 −𝒇 𝒂 𝒉

29. As ℎ (our interval width) approaches 0, the slopes of the secant lines approach the slope of the tangent line. With limits, we write the tangent slope like this: 𝐥𝐢𝐦 𝒉→𝟎 𝒇 𝒂+𝒉 −𝒇 𝒂 𝒉

30. 𝐥𝐢𝐦 𝒉→𝟎 𝒇 𝒂+𝒉 −𝒇 𝒂 𝒉 The name given to this particular limit is the ___________ of 𝑓 at the point 𝑎. It is written 𝒇′(𝒂), and is read “𝑓 prime of 𝑎”.

31. 𝐥𝐢𝐦 𝒉→𝟎 𝒇 𝒂+𝒉 −𝒇 𝒂 𝒉 The name given to this particular limit is the ___________ of 𝑓 at the point 𝑎. It is written 𝒇′(𝒂), and is read “𝑓 prime of 𝑎”.
derivative

32. 𝐥𝐢𝐦 𝒉→𝟎 𝒇 𝒂+𝒉 −𝒇 𝒂 𝒉 The name given to this particular limit is the ___________ of 𝑓 at the point 𝑎. It is written 𝒇′(𝒂), and is read “𝑓 prime of 𝑎”.
derivative

33. So, 𝐥𝐢𝐦 𝒉→𝟎 𝒇 𝒂+𝒉 −𝒇 𝒂 𝒉 has three interpretations: 1
So, 𝐥𝐢𝐦 𝒉→𝟎 𝒇 𝒂+𝒉 −𝒇 𝒂 𝒉 has three interpretations: 1. The instantaneous rate of change of 𝑓 with respect to 𝑥 at 𝑥=𝑎. 2. The slope of the tangent line at 𝑥=𝑎. 3. The derivative 𝑓′(𝑎).

34. So, 𝐥𝐢𝐦 𝒉→𝟎 𝒇 𝒂+𝒉 −𝒇 𝒂 𝒉 has three interpretations: 1
So, 𝐥𝐢𝐦 𝒉→𝟎 𝒇 𝒂+𝒉 −𝒇 𝒂 𝒉 has three interpretations: 1. The instantaneous rate of change of 𝑓 with respect to 𝑥 at 𝑥=𝑎. 2. The slope of the tangent line at 𝑥=𝑎. 3. The derivative 𝑓′(𝑎).

35. So, 𝐥𝐢𝐦 𝒉→𝟎 𝒇 𝒂+𝒉 −𝒇 𝒂 𝒉 has three interpretations: 1
So, 𝐥𝐢𝐦 𝒉→𝟎 𝒇 𝒂+𝒉 −𝒇 𝒂 𝒉 has three interpretations: 1. The instantaneous rate of change of 𝑓 with respect to 𝑥 at 𝑥=𝑎. 2. The slope of the tangent line at 𝑥=𝑎. 3. The derivative 𝑓′(𝑎).

36. So, 𝐥𝐢𝐦 𝒉→𝟎 𝒇 𝒂+𝒉 −𝒇 𝒂 𝒉 has three interpretations: 1
So, 𝐥𝐢𝐦 𝒉→𝟎 𝒇 𝒂+𝒉 −𝒇 𝒂 𝒉 has three interpretations: 1. The instantaneous rate of change of 𝑓 with respect to 𝑥 at 𝑥=𝑎. 2. The slope of the tangent line at 𝑥=𝑎. 3. The derivative 𝑓′(𝑎).

37. Ex 2. Find the slope of the tangent line to the curve 𝑓 𝑥 =5− 𝑥 2 at the point 𝑃(1,4). Now find an equation of the tangent line at 𝑃.

38. Ex 2. Find the slope of the tangent line to the curve 𝑓(𝑥)=5− 𝑥 2 at the point 𝑃(1,4). Now find an equation of the tangent line at 𝑃.

39. Ex 2. Find the slope of the tangent line to the curve 𝑓(𝑥)=5− 𝑥 2 at the point 𝑃(1,4). Now find an equation of the tangent line at 𝑃.

40. Ex 2. Find the slope of the tangent line to the curve 𝑓(𝑥)=5− 𝑥 2 at the point 𝑃(1,4). Now find an equation of the tangent line at 𝑃.

41. Ex 2. Find the slope of the tangent line to the curve 𝑓(𝑥)=5− 𝑥 2 at the point 𝑃(1,4). Now find an equation of the tangent line at 𝑃.

42. Ex 2. Find the slope of the tangent line to the curve 𝑓(𝑥)=5− 𝑥 2 at the point 𝑃(1,4). Now find an equation of the tangent line at 𝑃.
Find 𝑓 ′ __________

43. Ex 2. Find the slope of the tangent line to the curve 𝑓(𝑥)=5− 𝑥 2 at the point 𝑃(1,4). Now find an equation of the tangent line at 𝑃.
Find 𝑓 ′ __________ −𝟐

44. Ex 2. Find the slope of the tangent line to the curve 𝑓(𝑥)=5− 𝑥 2 at the point 𝑃(1,4). Now find an equation of the tangent line at 𝑃.
Find 𝑓 ′ __________ −𝟐
Find the rate at which 𝑓 𝑥
is changing with respect
to 𝑥 at 𝑥=1. ___________

45. Ex 2. Find the slope of the tangent line to the curve 𝑓(𝑥)=5− 𝑥 2 at the point 𝑃(1,4). Now find an equation of the tangent line at 𝑃.
Find 𝑓 ′ __________ −𝟐
Find the rate at which 𝑓 𝑥
is changing with respect
to 𝑥 at 𝑥=1. ___________ −𝟐

46. The Derivative as a Function
We can make a derivative function by letting the 𝑥-value be a variable, rather than a specific 𝑥=𝑎. 𝒇 ′ 𝒙 = 𝐥𝐢𝐦 𝒉→𝟎 𝒇 𝒙+𝒉 −𝒇 𝒙 𝒉 𝑓 is _____________ at 𝑥 if 𝑓′(𝑥) exists. ______________ is the process of calculating the derivative.

47. The Derivative as a Function
We can make a derivative function by letting the 𝑥-value be a variable, rather than a specific 𝑥=𝑎. 𝒇 ′ 𝒙 = 𝐥𝐢𝐦 𝒉→𝟎 𝒇 𝒙+𝒉 −𝒇 𝒙 𝒉 𝑓 is _____________ at 𝑥 if 𝑓′(𝑥) exists. ______________ is the process of calculating the derivative.
differentiable

48. The Derivative as a Function
We can make a derivative function by letting the 𝑥-value be a variable, rather than a specific 𝑥=𝑎. 𝒇 ′ 𝒙 = 𝐥𝐢𝐦 𝒉→𝟎 𝒇 𝒙+𝒉 −𝒇 𝒙 𝒉 𝑓 is _____________ at 𝑥 if 𝑓′(𝑥) exists. ______________ is the process of calculating the derivative.
differentiable
Differentiation

49. The following directions are all the same: “Differentiate 𝑓 𝑥 = 𝑥 2
The following directions are all the same: “Differentiate 𝑓 𝑥 = 𝑥 2 .” “Find the derivative of 𝑓 𝑥 = 𝑥 2 .” “Find 𝑓 ′ 𝑥 when 𝑓 𝑥 = 𝑥 2 .” “Find the slope of the tangent line of 𝑓 𝑥 = 𝑥 2 .” “Find the instantaneous rate of change of 𝑓 𝑥 = 𝑥 2 .”

50. The following directions are all the same: “Differentiate 𝑓 𝑥 = 𝑥 2
The following directions are all the same: “Differentiate 𝑓 𝑥 = 𝑥 2 .” “Find the derivative of 𝑓 𝑥 = 𝑥 2 .” “Find 𝑓 ′ 𝑥 when 𝑓 𝑥 = 𝑥 2 .” “Find the slope of the tangent line of 𝑓 𝑥 = 𝑥 2 .” “Find the instantaneous rate of change of 𝑓 𝑥 = 𝑥 2 .”

51. The following directions are all the same: “Differentiate 𝑓 𝑥 = 𝑥 2
The following directions are all the same: “Differentiate 𝑓 𝑥 = 𝑥 2 .” “Find the derivative of 𝑓 𝑥 = 𝑥 2 .” “Find 𝑓 ′ 𝑥 when 𝑓 𝑥 = 𝑥 2 .” “Find the slope of the tangent line of 𝑓 𝑥 = 𝑥 2 .” “Find the instantaneous rate of change of 𝑓 𝑥 = 𝑥 2 .”

52. The following directions are all the same: “Differentiate 𝑓 𝑥 = 𝑥 2
The following directions are all the same: “Differentiate 𝑓 𝑥 = 𝑥 2 .” “Find the derivative of 𝑓 𝑥 = 𝑥 2 .” “Find 𝑓 ′ 𝑥 when 𝑓 𝑥 = 𝑥 2 .” “Find the slope of the tangent line of 𝑓 𝑥 = 𝑥 2 .” “Find the instantaneous rate of change of 𝑓 𝑥 = 𝑥 2 .”

53. The following directions are all the same: “Differentiate 𝑓 𝑥 = 𝑥 2
The following directions are all the same: “Differentiate 𝑓 𝑥 = 𝑥 2 .” “Find the derivative of 𝑓 𝑥 = 𝑥 2 .” “Find 𝑓 ′ 𝑥 when 𝑓 𝑥 = 𝑥 2 .” “Find the slope of the tangent line of 𝑓 𝑥 = 𝑥 2 .” “Find the instantaneous rate of change of 𝑓 𝑥 = 𝑥 2 .”

54. Ex 3. Find the derivative of 𝑓 𝑥 = 𝑥 3 using the limit definition.

55. Ex 4. Find the derivative of 𝑓 𝑥 = 𝑥 𝑥−3 using the limit definition
Ex 4. Find the derivative of 𝑓 𝑥 = 𝑥 𝑥−3 using the limit definition. What is the slope of the tangent to the curve 𝑦= 𝑥 𝑥−3 at 𝑥=1? Find the rate at which 𝑓 𝑥 = 𝑥 𝑥−3 is changing with respect to 𝑥 at 𝑥=1.

56. Ex 4. Find the derivative of 𝑓 𝑥 = 𝑥 𝑥−3 using the limit definition
Ex 4. Find the derivative of 𝑓 𝑥 = 𝑥 𝑥−3 using the limit definition. What is the slope of the tangent to the curve 𝑦= 𝑥 𝑥−3 at 𝑥=2? Find the rate at which 𝑓 𝑥 = 𝑥 𝑥−3 is changing with respect to 𝑥 at 𝑥=1.

57. Ex 4. Find the derivative of 𝑓 𝑥 = 𝑥 𝑥−3 using the limit definition
Ex 4. Find the derivative of 𝑓 𝑥 = 𝑥 𝑥−3 using the limit definition. What is the slope of the tangent to the curve 𝑦= 𝑥 𝑥−3 at 𝑥=2? Find the rate at which 𝑓 𝑥 = 𝑥 𝑥−3 is changing with respect to 𝑥 at 𝑥=2.

58. Ex 4. Find the derivative of 𝑓 𝑥 = 𝑥 𝑥−3 using the limit definition
Ex 4. Find the derivative of 𝑓 𝑥 = 𝑥 𝑥−3 using the limit definition. What is the slope of the tangent to the curve 𝑦= 𝑥 𝑥−3 at 𝑥=2? Find the rate at which 𝑓 𝑥 = 𝑥 𝑥−3 is changing with respect to 𝑥 at 𝑥=2. −𝟑

61. Ex 5. Find the derivative of 𝑓 𝑥 = 𝑥 2 +1 using the limit definition
Ex 5. Find the derivative of 𝑓 𝑥 = 𝑥 2 +1 using the limit definition. Now find the tangent line to the curve 𝑦= 𝑥 2 +1 at 𝑥=1.

62. Ex 5. Find the derivative of 𝑓 𝑥 = 𝑥 2 +1 using the limit definition
Ex 5. Find the derivative of 𝑓 𝑥 = 𝑥 2 +1 using the limit definition. Now find the tangent line to the curve 𝑦= 𝑥 2 +1 at 𝑥=1.

65. Note: There are many ways people write the derivative of 𝑦=𝑓(𝑥):
𝑓 ′ 𝑥 = 𝑦 ′ = 𝑑𝑦 𝑑𝑥 = 𝑑𝑓 𝑑𝑥 = 𝑑 𝑑𝑥 𝑓 𝑥 =𝐷 𝑓 𝑥 = 𝐷 𝑥 𝑓(𝑥)
And here’s what it looks like to plug a value into the derivative:
𝑓 ′ 2 = 𝑑𝑦 𝑑𝑥 𝑥=2

66. Note: There are many ways people write the derivative of 𝑦=𝑓(𝑥):
𝑓 ′ 𝑥 = 𝑦 ′ = 𝑑𝑦 𝑑𝑥 = 𝑑𝑓 𝑑𝑥 = 𝑑 𝑑𝑥 𝑓 𝑥 =𝐷 𝑓 𝑥 = 𝐷 𝑥 𝑓(𝑥)
And here’s what it looks like to plug a value into the derivative:
𝑓 ′ 2 = 𝑑𝑦 𝑑𝑥 𝑥=2

67. One way to help ground your understanding of the derivative is to think about position vs. velocity.

68. Note that velocity is the rate at which position changes
Note that velocity is the rate at which position changes. That is, 𝑣 𝑡 = 𝑠 ′ 𝑡 . This also means that 𝑣 𝑡 is the slope of the tangent line of 𝑠(𝑡).

69. Note that velocity is the rate at which position changes
Note that velocity is the rate at which position changes. That is, 𝑣 𝑡 = 𝑠 ′ 𝑡 . This also means that 𝑣 𝑡 is the slope of the tangent line of 𝑠(𝑡).

70. Note that velocity is the rate at which position changes
Note that velocity is the rate at which position changes. That is, 𝑣 𝑡 = 𝑠 ′ 𝑡 . This also means that 𝑣 𝑡 is the slope of the tangent line of 𝑠(𝑡).

71. Ex 6. Suppose a robot moves back and forth along a line, and that the position of the robot (in meters) over time (in seconds) is given by the function 𝑠(𝑡) to the right. a. Graph the velocity function 𝑣 𝑡 .

78. Find 𝑠 ′ __________
Find 𝑠 ′ __________
Find 𝑠 ′ __________
Find 𝑣 7 . __________

79. 𝟎 m/s Find 𝑠 ′ 1 . __________ Find 𝑠 ′ 5 . __________

80. 𝟎 m/s −𝟐 m/s Find 𝑠 ′ 1 . __________ Find 𝑠 ′ 5 . __________

81. 𝟎 m/s −𝟐 m/s DNE Find 𝑠 ′ 1 . __________ Find 𝑠 ′ 5 . __________

82. 𝟎 m/s −𝟐 m/s DNE 𝟏 𝟐 m/s Find 𝑠 ′ 1 . __________

83. Here’s an example comparing 𝑓 𝑥 and 𝑓 ′ 𝑥 .

88. Note that 𝑓 ′ 𝑎 = lim ℎ→0 𝑓 𝑎+ℎ −𝑓 𝑎 ℎ only exists if both of the following limits exist and are equal: lim ℎ→ 0 − 𝑓 𝑎+ℎ −𝑓 𝑎 ℎ and lim ℎ→ 0 + 𝑓 𝑎+ℎ −𝑓 𝑎 ℎ
Left-hand derivative at 𝒙=𝒂
Right-hand derivative at 𝒙=𝒂

89. Note that 𝑓 ′ 𝑎 = lim ℎ→0 𝑓 𝑎+ℎ −𝑓 𝑎 ℎ only exists if both of the following limits exist and are equal: lim ℎ→ 0 − 𝑓 𝑎+ℎ −𝑓 𝑎 ℎ and lim ℎ→ 0 + 𝑓 𝑎+ℎ −𝑓 𝑎 ℎ
Left-hand derivative at 𝒙=𝒂
Right-hand derivative at 𝒙=𝒂

90. Note that 𝑓 ′ 𝑎 = lim ℎ→0 𝑓 𝑎+ℎ −𝑓 𝑎 ℎ only exists if both of the following limits exist and are equal: lim ℎ→ 0 − 𝑓 𝑎+ℎ −𝑓 𝑎 ℎ and lim ℎ→ 0 + 𝑓 𝑎+ℎ −𝑓 𝑎 ℎ
Left-hand derivative at 𝒙=𝒂
Right-hand derivative at 𝒙=𝒂

91. So, derivatives won’t exist at the following kinds of places:
corner
cusp
vertical tangent
discontinuity

92. So, derivatives won’t exist at the following kinds of places:
corner
cusp
vertical tangent
discontinuity

93. So, derivatives won’t exist at the following kinds of places:
corner
cusp
vertical tangent
discontinuity

94. So, derivatives won’t exist at the following kinds of places:
corner
cusp
vertical tangent
discontinuity

95. So, derivatives won’t exist at the following kinds of places:
corner
cusp
vertical tangent
discontinuity

96. Theorem: If 𝑓 has a derivative at 𝑥=𝑎, then 𝑓 is continuous at 𝑥=𝑎
Theorem: If 𝑓 has a derivative at 𝑥=𝑎, then 𝑓 is continuous at 𝑥=𝑎. (That is, differentiable functions are continuous. And if a function is not continuous at a point, then it is not differentiable there.)

97. Proof: Suppose that 𝑓 is differentiable at 𝑥=𝑎
Proof: Suppose that 𝑓 is differentiable at 𝑥=𝑎. Then lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 =𝐿 for some real number 𝐿. Also, lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 = lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 ⋅ 𝑥−𝑎 = lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 ⋅ lim 𝑥→𝑎 𝑥−𝑎 =𝐿⋅0=0 Thus, we have: lim 𝑥→𝑎 𝑓 𝑥 = lim 𝑥→𝑎 𝑓 𝑎 + 𝑓 𝑥 −𝑓 𝑎 = lim 𝑥→𝑎 𝑓 𝑎 + lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 =𝑓 𝑎 And so, 𝑓 is continuous at 𝑥=𝑎. ∎

98. Proof: Suppose that 𝑓 is differentiable at 𝑥=𝑎
Proof: Suppose that 𝑓 is differentiable at 𝑥=𝑎. Then lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 =𝐿 for some real number 𝐿. Also, lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 = lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 ⋅ 𝑥−𝑎 = lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 ⋅ lim 𝑥→𝑎 𝑥−𝑎 =𝐿⋅0=0 Thus, we have: lim 𝑥→𝑎 𝑓 𝑥 = lim 𝑥→𝑎 𝑓 𝑎 + 𝑓 𝑥 −𝑓 𝑎 = lim 𝑥→𝑎 𝑓 𝑎 + lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 =𝑓 𝑎 And so, 𝑓 is continuous at 𝑥=𝑎. ∎

99. Proof: Suppose that 𝑓 is differentiable at 𝑥=𝑎
Proof: Suppose that 𝑓 is differentiable at 𝑥=𝑎. Then lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 =𝐿 for some real number 𝐿. Also, lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 = lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 ⋅ 𝑥−𝑎 = lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 ⋅ lim 𝑥→𝑎 𝑥−𝑎 =𝐿⋅0=0 Thus, we have: lim 𝑥→𝑎 𝑓 𝑥 = lim 𝑥→𝑎 𝑓 𝑎 + 𝑓 𝑥 −𝑓 𝑎 = lim 𝑥→𝑎 𝑓 𝑎 + lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 =𝑓 𝑎 And so, 𝑓 is continuous at 𝑥=𝑎. ∎

100. Proof: Suppose that 𝑓 is differentiable at 𝑥=𝑎
Proof: Suppose that 𝑓 is differentiable at 𝑥=𝑎. Then lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 =𝐿 for some real number 𝐿. Also, lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 = lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 ⋅ 𝑥−𝑎 = lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 ⋅ lim 𝑥→𝑎 𝑥−𝑎 =𝐿⋅0=0 Thus, we have: lim 𝑥→𝑎 𝑓 𝑥 = lim 𝑥→𝑎 𝑓 𝑎 + 𝑓 𝑥 −𝑓 𝑎 = lim 𝑥→𝑎 𝑓 𝑎 + lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 =𝑓 𝑎 And so, 𝑓 is continuous at 𝑥=𝑎. ∎

101. Proof: Suppose that 𝑓 is differentiable at 𝑥=𝑎
Proof: Suppose that 𝑓 is differentiable at 𝑥=𝑎. Then lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 =𝐿 for some real number 𝐿. Also, lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 = lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 ⋅ 𝑥−𝑎 = lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 ⋅ lim 𝑥→𝑎 𝑥−𝑎 =𝐿⋅0=0 Thus, we have: lim 𝑥→𝑎 𝑓 𝑥 = lim 𝑥→𝑎 𝑓 𝑎 + 𝑓 𝑥 −𝑓 𝑎 = lim 𝑥→𝑎 𝑓 𝑎 + lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 =𝑓 𝑎 And so, 𝑓 is continuous at 𝑥=𝑎. ∎

102. Proof: Suppose that 𝑓 is differentiable at 𝑥=𝑎
Proof: Suppose that 𝑓 is differentiable at 𝑥=𝑎. Then lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 =𝐿 for some real number 𝐿. Also, lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 = lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 ⋅ 𝑥−𝑎 = lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 ⋅ lim 𝑥→𝑎 𝑥−𝑎 =𝐿⋅0=0 Thus, we have: lim 𝑥→𝑎 𝑓 𝑥 = lim 𝑥→𝑎 𝑓 𝑎 + 𝑓 𝑥 −𝑓 𝑎 = lim 𝑥→𝑎 𝑓 𝑎 + lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 =𝑓 𝑎 And so, 𝑓 is continuous at 𝑥=𝑎. ∎

103. Proof: Suppose that 𝑓 is differentiable at 𝑥=𝑎
Proof: Suppose that 𝑓 is differentiable at 𝑥=𝑎. Then lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 =𝐿 for some real number 𝐿. Also, lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 = lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 ⋅ 𝑥−𝑎 = lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 ⋅ lim 𝑥→𝑎 𝑥−𝑎 =𝐿⋅0=0 Thus, we have: lim 𝑥→𝑎 𝑓 𝑥 = lim 𝑥→𝑎 𝑓 𝑎 + 𝑓 𝑥 −𝑓 𝑎 = lim 𝑥→𝑎 𝑓 𝑎 + lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 =𝑓 𝑎 And so, 𝑓 is continuous at 𝑥=𝑎. ∎

104. Proof: Suppose that 𝑓 is differentiable at 𝑥=𝑎
Proof: Suppose that 𝑓 is differentiable at 𝑥=𝑎. Then lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 =𝐿 for some real number 𝐿. Also, lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 = lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 ⋅ 𝑥−𝑎 = lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 ⋅ lim 𝑥→𝑎 𝑥−𝑎 =𝐿⋅0=0 Thus, we have: lim 𝑥→𝑎 𝑓 𝑥 = lim 𝑥→𝑎 𝑓 𝑎 + 𝑓 𝑥 −𝑓 𝑎 = lim 𝑥→𝑎 𝑓 𝑎 + lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 =𝑓 𝑎 And so, 𝑓 is continuous at 𝑥=𝑎. ∎

105. Proof: Suppose that 𝑓 is differentiable at 𝑥=𝑎
Proof: Suppose that 𝑓 is differentiable at 𝑥=𝑎. Then lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 =𝐿 for some real number 𝐿. Also, lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 = lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 ⋅ 𝑥−𝑎 = lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 𝑥−𝑎 ⋅ lim 𝑥→𝑎 𝑥−𝑎 =𝐿⋅0=0 Thus, we have: lim 𝑥→𝑎 𝑓 𝑥 = lim 𝑥→𝑎 𝑓 𝑎 + 𝑓 𝑥 −𝑓 𝑎 = lim 𝑥→𝑎 𝑓 𝑎 + lim 𝑥→𝑎 𝑓 𝑥 −𝑓 𝑎 =𝑓 𝑎 And so, 𝑓 is continuous at 𝑥=𝑎. ∎