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## Algebra II (2018 edition)

### Unit 4: Lesson 14

Introduction to symmetry of functions# Even and odd functions: Graphs

CCSS.Math:

Sal picks the function that is odd among three functions given by their graphs. Created by Sal Khan.

## Want to join the conversation?

- So what actually happened at3:43?(118 votes)
- sal said a car crashed. I thought he fell off his chair.(15 votes)

- I was wondering why the odd and even types of a function don't deal with symmetry over the x-axis? As in Odd asks whether the function is symmetrical with respect to the origin f(-x) =

-f(x), and Even is when the function is symmetrical with respect to the y-axis f(-x) = f(x), but why doesn't this deal with symmetry over the x-axis? Thank you:)(5 votes)- A curve that is symmetric over the x-axis isn't a function, since it fails the vertical line test.(7 votes)

- I am not sure if I understand the odd or even function because it's labeled at x=8 and x=-8. Is it because the function itself of -j(-a) would result in the opposite sign or something?(2 votes)
- Yes, that is the right mindset towards to understanding if the function is odd or even.

For it to be odd:

j(a) = -(j(a))

Rather less abstractly,**the function would**. So yes, if you were given a point (4,-8), reflecting off the x axis and the y axis, it would output: (-4,8)

both reflect off the y axis and the x axis, and it would still look the same

For it to be even:

j(a) = j(-a)

Less abstractly,**the function reflects off the y-axis and would still look the same as the original, non translated function**.(6 votes)

- In the beginning, is the answer is h(x) by odd?(3 votes)
- why was y=-x+4 not an odd function, i thought that a function is odd when its exponent is an odd number, please explain(2 votes)
- The function is odd if
`f(x) = -f(-x)`

. The rule of a thumb might be that if a function doesn't intercepts y at the origin, then it can't be odd, and`y = -x + 4`

is shifted up and has y-intercept at 4.

Now, evenness or oddness of functions is connected to the exponents, but the exponent has to be odd on**every**term. And that`4`

is actually`4*x^0`

, so it's a term with even exponent. And when you have a mixture of even and odd exponents, then the function as a whole ends up being neither even nor odd.(2 votes)

- Maybe the Autobots are fighting Megatron again. That explains the sound at3:43. Sal, you should probably run.(2 votes)
- What exactly does it mean for a function to be odd?(1 vote)
- A function f(x) is odd when f(-x)=-f(x) for every x. The graph of an odd function has a particular rotational symmetry, so the graph will look the same after you rotate it 180º about the origin.(3 votes)

- Do all odd functions cross the origin?(2 votes)
- Will an odd function always go through the origin?(1 vote)
- Yes because they must have symmetry around the origina. Tha's part of the definition of an odd function.(2 votes)

- wait so is f(x) = -x + 4 not an odd function because 4 = 4*x^0 and 0 is even?(1 vote)
- No, it's not odd because the graph does not pass thru the origin (0,0). Odd functions have symmetry with respect to the origin and where f(-x) = -f(x).

Hope this helps.(2 votes)

## Video transcript

Which of these functions is odd? And so let's remind
ourselves what it means for a
function to be odd. So I have a function--
well, they've already used f, g,
and h, so I'll use j. So function j is odd. If you evaluate j at some
value-- so let's say j of a. And if you evaluate that j at
the negative of that value, and if these two things are
the negative of each other, then my function is odd. If these two things
were the same-- if they didn't have this
negative here-- then it would be an even function. So let's see which of these
meet the criteria of being odd. So let's look at f of x. So we could pick a
particular point. So let's say when
x is equal to 2. So we get f of 2 is equal to 2. Now, what is f of negative 2? f of negative 2
looks like it is 6. f of negative 2 is equal to 6. So these aren't the
negative of each other. In order for this to
be odd, f of negative 2 would have had to be equal to
the negative of this, would have had to be
equal to negative 2. So f of x is definitely not odd. So all I have to do is
find even one case that violated this
constraint to be odd. And so I can say it's
definitely not odd. Now let's look at g of x. So I could use the same-- let's
see, when x is equal to 2, we get g of 2 is
equal to negative 7. Now let's look at
when g is negative 2. So we get g of negative 2
is also equal to negative 7. So here we have a
situation-- and it looks like that's the case for
any x we pick-- that g of x is going to be equal
to g of negative x. So g of x is equal
to g of negative x. It's symmetric
around the y-- or I should say the vertical
axis-- right over here. So g of x is even, not odd. So which of these
functions is odd? Definitely not g of x. So our last hope is h of x. Let's see if h of x seems
to meet the criteria. I'll do it in this green color. So if we take h of 1-- and we
can look at it even visually. So h of 1 gets us right
over here. h of negative 1 seems to get us an equal amount,
an equal distance, negative. So it seems to fit for 1. For 2-- well, 2
is at the x-axis. But that's definitely h of 2
is 0. h of negative 2 is 0. But those are the
negatives of each other. 0 is equal to negative 0. If we go to, say, h of 4, h
of 4 is this negative number. And h of negative 4 seems
to be a positive number of the same magnitude. So once again, this is
the negative of this. So it looks like this is
indeed an odd function. And another way to visually
spot an odd function is a function-- it's going
to go through the origin, and you could essentially
flip it over on both axes. So if you flip this, the right
half, over the left half, and then flip that over
the horizontal axis, you are going to get
this right over here. So you see here we're
going up and to the right. Here we're going to go
down and to the left. And then you curve
right over there. You curve up just like that. But the easiest way to test
it is just to do what we did, look at a given x. So for example, when x
is equal to 8, h of 8 looks like this
number right around 8. h of negative 8 looks like it's
pretty close to negative 8. So they seem to be the
negative of each other. It sounds like a car crash
just happened outside. Anyway, hopefully
you enjoyed that. Not the car crash,
the math problem.