Section 1.4 – Continuity and One-Sided Limits
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Transcript of Section 1.4 – Continuity and One-Sided Limits
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Section 1.4 – Continuity and One-Sided Limits
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ExampleFind values of a and b that makes f(x) continuous.
f x ax 3 if x 58 if x 5
x 2 +bx +1 if x 5
When x=5, all three pieces must have a limit of 8.
5a 5
ax 3 8
a 5 3 8
a 1
5b 28 8
x 2 bx 3 8
5 2 b 5 3 8
5b 20
b 4
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Continuity at a Point
A function f is continuous at c if the following three conditions are met:
1. is defined.
2. exists.
3.
( )f c
lim ( )x c
f x
lim ( ) ( )x c
f x f c
c
L
f(x)
x
For every question of this type, you need (1), (2), (3), conclusion.
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Example 1
Show is continuous at x = 0.
f x 2 1 x 2
1. f 0
2 1 02
1The function is clearly
defined at x= 0
2. limx 0
2 1 x 2
2 1 02
1With direct
substitution the limit clearly exists at x=0
3. f 0 limx 0
2 1 x 2The value of the
function clearly equals the limit at x=0
f is continuous at x = 0
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Example 2
Show is not continuous at x = 2. 8 1 if 210 if 2
x xf x
x
1. 2f 10 The function is clearly 10 at x = 2
2
lim 8 1x
x
8 2 1 15
With direct substitution the limit clearly exists at x=0
2
3. 2 lim x
f f x
The value of the function clearly does not equal the limit at x=2
f is not continuous at x = 2
The behavior as x approaches 2 is dictated by 8x-1
2
2. lim x
f x
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DiscontinuityIf f is not continuous at a, we say f is discontinuous
at a, or f has a discontinuity at a.
Typically a hole in the curve
Step/Gap Asymptote
Types Of DiscontinuitiesRemovable
Able to remove the “hole” by defining f at one point
Non-RemovableNOT able to remove the “hole” by defining f at
one point
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ExampleFind the x-value(s) at which is not
continuous. Which of the discontinuities are removable?
22 153( ) x x
xf x
There is a discontinuity at x=-3 because this makes the
denominator zero.
If f can be reduced, then the discontinuity is removable:
2 5 33
x xxf x
2 5 33
x xx
2 5x This is the
same function as f except at
x=-3f has a removable
discontinuity at x = -3
Notice that:
003f
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Indeterminate Form: 0/0
Let:
g xf x
h x
If:
00
g cf c
h c
Then f(x) has a removable discontinuity at x=c.
If f(x) has a removable discontinuity at x=c.
Then the limit of f(x) at x=c exists.
c
L
f(x)
x
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One-Sided Limits: Left-HandIf f(x) becomes arbitrarily close to a single REAL
number L as x approaches c from values less than c, the left-hand limit is L.
The limit of f(x)…
as x approaches c from the left…
is L.Notation:
c
L
f(x)
x
lim ( )x c
f x L
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lim ( )x c
f x L
One-Sided Limits: Right-HandIf f(x) becomes arbitrarily close to a single REAL
number L as x approaches c from values greater than c, the right-hand limit is L.
The limit of f(x)…
as x approaches c from the right…
is L.Notation:
c
L
f(x)
x
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Example 1Evaluate the following limits for
f x x
limx 1
f x
limx 1
f x
limx 1
f x
1
0
DNE
3.5
limx
f x
3.5
limx
f x
3.5
limx
f x
3
3
3
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Example 2
Analytically find . 12
24
3 if 4lim if
if 4x
x xf x f x
x x
12If is approaching 4 from the left, the function is defined by 3x x
4
limx
f x
124
lim 3x
x 1
2 4 3
4
Therefore lim 5x
f x
5
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The Existence of a Limit
Let f be a function and let c be real numbers. The limit of f(x) as x approaches c is L if and only if
lim ( ) lim ( )x c x c
f x L f x
c
L
f(x)
x
Right-Hand LimitLeft-Hand Limit =A limit exists if…
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Example 1Analytically show that .
limx 2
x 2 1 1
Evaluate the right hand limit at 2 :x
limx 2
x 2 1
limx 2
x 2 1
2 2 1
2 1
x
if 2if 2
xx
1
Evaluate the left hand limit at 2 :x
limx 2
x 2 1
limx 2
x 2 1
2 2 1
1
Therefore limx 2
x 2 1 1
Use when x>2
Use when x<2You must use the piecewise equation:
2 +12 +1
xx
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Example2Analytically show that is continuous at x = -
1. = 1f x x
Evaluate the right hand limit at 1:x
1lim 1
xx
1lim 1
xx
1 1
f x
if 1if 1
xx
0
Evaluate the left hand limit at 1:x
1lim 1
xx
1lim 1
xx
1 1 0
Therefore is continuous at 1f x x
Use when x>-1
Use when x<-1
You must use the piecewise equation:
11
xx
1. 1f 1 1 0
1
2. Find limx
f x
0
1
3. 1 lim x
f f x
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Continuity on a Closed Interval
A function f is continuous on [a, b] if it is continuous on (a, b) and
lim ( ) ( ) lim ( ) ( )x a x b
f x f a and f x f b
a
f(a)
f(b)
xb
Must have closed dots on the endpoints.
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t x
Example 1Use the graph of t(x) to determine the intervals on
which the function is continuous.
6, 3 3,0 0,2 2,5 5,6
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Example 2Discuss the continuity of
f x 1 1 x 2
The domain of f is [-1,1]. From our limit properties, we can say it is continuous on (-
1,1)
By direct substitution:
limx 1
f x
1 1 1 2
1
f 1
limx 1
f x
1 1 1 2
1
f 1
Is the middle is continuous?
Are the one-sided limits of the endpoints equal to the functional value?
f is continuous on [-1,1]
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Properties of ContinuityIf b is a real number and f and g are continuous at x = c,
then following functions are also continuous at c:1. Scalar Multiple:
2. Sum/Difference:
3. Product:
4. Quotient: if
5. Composition:
Example: Since are continuous, is continuous too.
f og x
f x g x
f x g x
b f x
f x g x
g c 0
f x 2x and g x x 2
h x 2x x 2
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Intermediate Value TheoremIf f is continuous on the closed interval [a, b] and k
is any number between f(a) and f(b), then there is at least one number c in [a, b] such that:
( )f c k
a b
f(a)
f(b)
k
c
This theorem does NOT find
the value of c. It just proves it
exists.
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Free Response Exam 2007
1h 1 6f g 2 6f 9 6 3
3h 3 6f g 4 6f 1 6 7
Since h(3) < -5 < h(1) and h is continuous, by the IVT, there
exists a value r, 1 < r < 3, such that h(r) = -5.
Notice how every part of the theorem is
discussed (values of the function AND continuity).
We will learn later that this implies continuity.
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ExampleUse the intermediate value theorem to show
has at least one root.
f x 4 x 3 6x 2 3x 2
f 0 4 0 3 6 0 2 3 0 2
2
f 2 4 2 3 6 2 2 3 2 2
12Find an output greater than zero
Find an output less than zero
Since f(0) < 0 and f(2) > 0
There must be some c such that f(c) = 0 by the IVTThe IVT can be used since f
is continuous on [-∞,∞].
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ExampleShow that has at least one solution on the
interval .
cos x x 3 x
f 4 cos
4 4 3
4
1.008
f 2 cos
2 2 3
2
2.305Find an output less than zero
Find an output greater than zero
Since and
There must be some c such that cos(c) = c3 - c by the IVT
The IVT can be used since the left and right side are
both continuous on [-∞,∞].
4 ,
2 Solve the equation for zero.
cos x x 3 x 0
f x
f 4 0
f 2 0