limit & derivative problems by anurag tyagi classes (atc)
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Limit & Derivative Problems
Problem… Answer and Work…
1.
1. 2
2 4
3 7 11lim
2 4 8x
x x
x x x
1
2
4 4 4
2 4
4 4 4
3 7 110
lim 082 4 8x
x x
x x xx x x
x x x
Limit & Derivative Problems
Problem… Answer and Work…
2.
2.
16
16lim
4x
x
x
2
16 16
4 4lim lim 4 16 4 8
4x x
x xx
x
Limit & Derivative Problems
Problem… Answer and Work…
3.
3. 3
2
8lim
2x
xx
3
2
2
2 2
2
2 2 4lim
2lim 2 4 2 2 2 4
4 4 4
12
x
x
x x x
xx x
Limit & Derivative Problems
Problem… Answer and Work…
4. Consider the function given by
Is f(x) continuous at x=1? Justify.
4.
2 3, x 1( )
3x, x > 1
xf x
4
12
1
1
Does lim ( ) (1)?
lim ( ) 1 3 4
lim ( ) 3 1 3
lim not equal to each other,
theref ore,
not continuous at x = 1
x
x
x
f x f
f x
f x
Limit & Derivative Problems
Problem… Answer and Work…
5. Is the function given by
continuous for all x? If not, where are the discontinuities? Are they removable?
5.
2, x 3( )
6 - x, x 3
xh x
5
3
3
3
3 3
lim ( ) ( )?
lim ( ) 3 2 5
lim ( ) 6 3 3
not continuous at x = 3
lim ( ) lim ( )
shows that the discontinuity
is not removable
x
x
x
x x
h x h x
h x
h x
h x h x
Limit & Derivative Problems
Problem… Answer and Work…6. Let the piecewise function f be
defined as follows:
Which of the following is true about the function f?
I. f(2) = 2
II.
III. f(x) is continuous at x = 2
A. I only
B. III only
C. I and II only
D. I and III only
E. I, II, and III
6. Test: f(2) = 2? Yes, so I is true
Test:
Test: f(x) is continuous at x = 2?
Does the lim f(x) = f(2)?
4 is not equal to 2
No, so III is false
Answer is A) I only
2 4, f or x 2
( ) 22, f or x = 2
xf x x
6
2lim ( ) 2x
f x
2
22
22
lim ( ) 2?
( 2)( 2)lim ( ) lim
( 2)lim ( ) lim 2 4
No, I I is f alse
x
xx
xx
f x
x xf x
xf x x
Limit & Derivative Problems
Problem… Answer and Work…
7.
What is the value of a for which f(x) is continuous for all values of x?
A. -2
B. -1
C. 0
D. ½
E. 1
7. To be continuous at x = 1
7
2
1, x 1I f ( )
3 , x > 1
xf x
ax
1 12
1 12
2
lim ( ) lim ( )
lim 1 lim3
2 = 3 + ax
2 = 3 + a(1)
2 = 3 + a
-1 = a
x x
x x
f x f x
x ax
Limit & Derivative Problems
Problem… Answer and Work…
8. Find the cartesian coordinates of the point on the graph of
where the instantaneous rate of change of f is equal to 5
8.
to find y substitute x = ½ in the original function f(x)
Ans: (1/2, 11/4)
8
2( ) 3 2 1f x x x
2( ) 3 2 1f x x x
' ( ) 6 2
5 6 2
3 6
12
f x x
x
x
x
Limit & Derivative Problems
Problem… Answer and Work…
9. Which of the following directly describes the discontinuities associated with
a. A hole at x = 3, a vertical asymptote at x = 3
b. Holes at x = -3 and x = 3
c. A hole at x = 3, a vertical asymptote at x = -3
d. Vertical asymptotes at x = 3 and x = -3
e. No discontinuities
9.
Hole at x = 3 because we factored out (x – 3)
There is a vertical asymptote at x = -3
9
2
2
2 3( )
9
x xf x
x
( 3)( 1) 1( 3)( 3) 3x x xx x x
Limit & Derivative Problems
Problem… Answer and Work…10. Given the piecewise function
For what values of a and b is f(x) differentiable at x = 1?
A. a = 2 b = -3
B. a = 2 b = -2
C. a = -2 b = 1
D. a = 3 b = -1
E. a = 5 b = 8
10. Differentiability implies continuity
To be differentiable x = 1
Solve for a when b = 1
a – 1 = -3 a = -2 Ans: C
10
2
2 x 1( )
bx 1 x > 1
x af x
2
2
2 1 when x = 1
2(1) + a = b(1) 1
2 1
3
x a bx
a b
a b
2(2 ) ( 1)
2 2 when x = 1
2 2 (1)
1
d dx a bx
dx dxbx
b
b
Limit & Derivative Problems
Problem… Answer and Work…
11. Which of the following is (are) true about the function
I. It is continuous at x = 0II. It is differentiable at x = 0III.
A. I onlyB. II onlyC. I and III onlyD. II and III onlyE. I, II, III
11. Test 1: Continuous at x = 0
yes
Test 2: Differentiable at x = 0?
No
Test 3:
Yes
Ans: C
11
13( ) ?f x x
0lim ( ) 0x
f x
3( )f x x
2 3
2 3
1'( )
31
03
undefi ned at x = 0
f x x
x
0
0 0
lim ( ) 0?
lim ( ) 0 lim ( ) 0x
x x
f x
f x f x
Limit & Derivative Problems
Problem… Answer and Work…12. To apply either the Mean Value
Theorem or Rolle’s Theorem to a function f, certain requirements regarding the continuity and differentiability of the function must be met. Which of the following states the requirements correctly?
A. f is continuous on (a, b) and differentiable on (a, b)
B. f is continuous on (a, b) and differentiable on [a, b]
C. f is continuous on (a, b) and differentiable on [a, b)
D. f is continuous on [a, b] and differentiable on (a, b)
E. f is continuous on [a, b] and differentiable on [a, b]
12. Look at the definition of Rolle’s Theorem and the Mean Value Theorem
f is continuous on [a, b] and differentiable on (a, b)
Ans: D
12
Limit & Derivative Problems
Problem… Answer and Work…13. Let f be the function defined by
A. Determine the x and y intercepts, if any. Justify your answer.
13. A
13
2
1 1( ) 1f x
x x
2
2
2
2
2
2
2
1( )
10
0 1 no real solutions
n
x-intercept
y-intercep
o x-intercepts
0
t
0 1 undefi ned
0no y-intercepts
x xf x
x
x x
xx x
y
Limit & Derivative Problems
Problem… Answer and Work…13. Let f be the function defined by
B. Write an equation for each vertical and each horizontal asymptote. Justify your answer.
13. B
Vertical asymptote
Horizontal asymptote
14
2
1 1( ) 1f x
x x
2 0
0
x
x
2
2 2 2 2
2 20 0
2
11
lim lim 1
1
x x
x xx x x x x
x x
xy
Limit & Derivative Problems
Problem… Answer and Work…13. Let f be the function defined by
C. Determine the intervals on which f is increasing or decreasing. Justify your answer.
13. C
15
2
1 1( ) 1f x
x x
1 2
2 3
3
3
( ) 1
2'( ) 2
0 ( 2)
0 x = -2
Do a sign graph f or the critical points 0, -2
0 is a vertical asympt
Decreasi
ote
ng (- , -2) and (0, )
I ncreasing (-2, 0)
f x x x
xf x x x
x
x x
x
Limit & Derivative Problems
Problem… Answer and Work…13. Let f be the function defined by
D. Determine the relative minimum and maximum points, if any. Justify your answer.
13. D
Relative minimum occurs at x = -2
when x = -2
16
2
1 1( ) 1f x
x x
1 2( 2) 1 ( 2) ( 2)
1 1 31
2 4 43
( 2, )4
no maximum because at x = 0 which
is the vertical asymptote
f
Limit & Derivative Problems
Problem… Answer and Work…13. Let f be the function defined by
E. Determine the intervals on which f is concave up or concave down. Justify your answer.
13. E
17
2
1 1( ) 1f x
x x 2 3
3 4
4
' ( ) 2
"( ) 2 6
2 ( 3)
undefi ned at x=0 because
it is a vertical asymptote
0 = x+3
3
Do a sign graph using critical
Concaves d
points -3,
own (- , -3)
Concaves up (-3,0) (0, )
0
f x x x
f x x x
x x
x
Limit & Derivative Problems
Problem… Answer and Work…13. Let f be the function defined by
F. Determine any points of inflection
13. F
Point of inflection when x = 3
18
2
1 1( ) 1f x
x x
1 2
2
( 3) 1 ( 3) ( 3)
1 11
7( 3, )
9
3 ( 3)9 3 19 9 979
f
Limit & Derivative Problems
Problem… Answer and Work…14. On the interval [1, 3], what is the
average rate of change for the functions, if
14.
19
2( ) 3 4 ?s t t t
2
2
(3) (1)3 1
(3) 3(3) 4(3) 27 12 15
(1) 3(1) 4(1) 3 4 1
15 ( 1)8
3 1
s sAvg
s
s
Limit & Derivative Problems
Problem… Answer and Work…15. Is the function defined by
continuous at x = 4? Justify your answer.
15.
20
3, 3 x < 7( )
5, 7
xf x
x x
4
4
lim 3 4 3 1
(4) 4 3 1
lim ( ) (4)
x
x
x
f
f x f