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Where a function is not differentiable - Mathematics Video Lecture - Engineering Mathematics
FAQs on Where a function is not differentiable - Mathematics Video Lecture - Engineering Mathematics
| 1. What is the definition of differentiability in mathematics? |  |
Ans. In mathematics, a function is said to be differentiable at a point if its derivative exists at that point. The derivative of a function represents the rate at which the function is changing at a particular point.
| 2. Can a function be differentiable everywhere? | |
Ans. No, a function may not be differentiable everywhere. There are certain points or regions where a function may not have a derivative. These points are typically characterized by sharp corners, cusps, or discontinuities in the function.
| 3. What are some common examples of functions that are not differentiable? | |
Ans. Some common examples of functions that are not differentiable include absolute value function, step function, and functions with vertical tangents or sharp corners such as the function |x| at x=0.
| 4. How can we determine if a function is differentiable at a specific point? | |
Ans. To determine if a function is differentiable at a specific point, we need to check if the derivative of the function exists at that point. We can do this by finding the derivative of the function and evaluating it at the given point. If the derivative exists, then the function is differentiable at that point.
| 5. Can a function be continuous but not differentiable? | |
Ans. Yes, a function can be continuous but not differentiable. In fact, all differentiable functions are continuous, but not all continuous functions are differentiable. A function may be continuous if it has no abrupt jumps or breaks, but it may still have points where the derivative does not exist, making it non-differentiable at those points.
Text Transcript from Video
Consider f which is defined
for all real numbers.
At what arguments x is f
of x not differentiable?
So to think about
that, I'm actually
going to try to visualize what
f prime of x must look like.
So I'm going to do f prime
of x in this purple color.
So if we look at f
of x right over here,
it looks like its slope is
pretty much consistently
negative 2 over this interval
between x equals, I guess,
it's like negative 8
and 1/2 all the way
up to x equals negative 2.
It looks like the slope
is a constant negative 2.
So if I were to
draw its derivative,
its derivative would
look something like this.
Its derivative looks
something like this.
But then something interesting
happens at x equals negative 2.
Right as we cross x
equals negative 2,
it looks like the
slope goes from being
negative to being positive.
And it looks like
right out the get
go, if I were to estimate the
slope of its tangent line,
it starts changing.
It's not a line anymore.
It's a curve.
The slope of the tangent
line right at this point
looks like it's
around-- I don't know--
it looks like it's
around 3 and 1/2.
Because if I were to draw a
tangent line right over here,
it looks like if I move
1 in the x direction,
I move up about 3 and
1/2 in the y direction.
So I'm just trying to,
obviously, estimate it.
So it looks like the
slope goes up to 3 and 1/2
right when I cross that point.
And then the slope becomes
lower and lower and lower
all the way until I get to
this point right over here,
all the way until I
get to x equals 2.
And it looks like it continues
to get lower all the way
until you get to x equals 3.
So it looks like the
slope of the line
is-- it looks like it's getting
lower at a constant rate,
I guess I could say.
So it looks like
it's doing something
like this over this interval.
But then right as x crosses
3, this becomes a flat line.
The slope is 0 here.
So right as x crosses
3, the slope becomes 0.
So we immediately
see there are points
where it looks like
the slope jumps.
And at these points
we really don't
have a defined derivative.
The slope jumps there as well.
And so at what arguments
is f not differentiable?
Well, it's not differentiable
when x is equal to negative 2.
When x is equal to
negative 2, we really
don't have a slope there.
Remember, when
we're trying to find
the slope of the
tangent line, we
take the limit of the
slope of the secant line
between that point and some
other point on the curve.
If we did that as we
approached from the left,
it looks like the
derivative is negative 2.
If we do that from the right,
it looks like the derivative
is something like
positive 3 and 1/2.
And so we're not getting the
same limit of the secant line
as we approach from the left and
as we approach from the right.
And the same thing is
happening at x is equal to 3.
At x equals 3, as we
approach from the left,
the slope looks like
it is decreasing.
It is approaching-- I don't
know-- maybe around negative 1.
But as we approach
from the right
it looks like the slope is 0.
So we do not have the same
limit of the secant slope
as we approach from the
left- and right-hand sides.
So at both of these points
we see the derivative jump,
and it looks like f of
x is not differentiable.
for all real numbers.
At what arguments x is f
of x not differentiable?
So to think about
that, I'm actually
going to try to visualize what
f prime of x must look like.
So I'm going to do f prime
of x in this purple color.
So if we look at f
of x right over here,
it looks like its slope is
pretty much consistently
negative 2 over this interval
between x equals, I guess,
it's like negative 8
and 1/2 all the way
up to x equals negative 2.
It looks like the slope
is a constant negative 2.
So if I were to
draw its derivative,
its derivative would
look something like this.
Its derivative looks
something like this.
But then something interesting
happens at x equals negative 2.
Right as we cross x
equals negative 2,
it looks like the
slope goes from being
negative to being positive.
And it looks like
right out the get
go, if I were to estimate the
slope of its tangent line,
it starts changing.
It's not a line anymore.
It's a curve.
The slope of the tangent
line right at this point
looks like it's
around-- I don't know--
it looks like it's
around 3 and 1/2.
Because if I were to draw a
tangent line right over here,
it looks like if I move
1 in the x direction,
I move up about 3 and
1/2 in the y direction.
So I'm just trying to,
obviously, estimate it.
So it looks like the
slope goes up to 3 and 1/2
right when I cross that point.
And then the slope becomes
lower and lower and lower
all the way until I get to
this point right over here,
all the way until I
get to x equals 2.
And it looks like it continues
to get lower all the way
until you get to x equals 3.
So it looks like the
slope of the line
is-- it looks like it's getting
lower at a constant rate,
I guess I could say.
So it looks like
it's doing something
like this over this interval.
But then right as x crosses
3, this becomes a flat line.
The slope is 0 here.
So right as x crosses
3, the slope becomes 0.
So we immediately
see there are points
where it looks like
the slope jumps.
And at these points
we really don't
have a defined derivative.
The slope jumps there as well.
And so at what arguments
is f not differentiable?
Well, it's not differentiable
when x is equal to negative 2.
When x is equal to
negative 2, we really
don't have a slope there.
Remember, when
we're trying to find
the slope of the
tangent line, we
take the limit of the
slope of the secant line
between that point and some
other point on the curve.
If we did that as we
approached from the left,
it looks like the
derivative is negative 2.
If we do that from the right,
it looks like the derivative
is something like
positive 3 and 1/2.
And so we're not getting the
same limit of the secant line
as we approach from the left and
as we approach from the right.
And the same thing is
happening at x is equal to 3.
At x equals 3, as we
approach from the left,
the slope looks like
it is decreasing.
It is approaching-- I don't
know-- maybe around negative 1.
But as we approach
from the right
it looks like the slope is 0.
So we do not have the same
limit of the secant slope
as we approach from the
left- and right-hand sides.
So at both of these points
we see the derivative jump,
and it looks like f of
x is not differentiable.
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Oct 19, 2025
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