Hi everyone. Today we’re going to talk about

how to calculate combinations of two functions and then define the domain of those combinations.

To complete this problem, we will calculate our combinations, simplify and then define

the domain of each. Let’s take a look. In this particular problem, we’ve been given

the two functions f of x equals x cubed plus 2 x squared and g of x equals 3 x squared

minus 1 and asked to find the following four functions and their domains. So as you can

see, we’re going to be finding f plus g, f minus g, f times g and f divided by g and

we’re just talking here about combinations of functions. These are different combinations of f and

g. The first one is obviously just the sum of the functions, the difference of the functions,

the multiplication and the division of the functions or the product and the quotient

of the functions. So let’s go ahead and start with f plus

g of x and this is going to be really simple. When we add functions together like this to

find a combination of these functions, we will just add f and g together. So f here

is x cubed plus 2 x squared and g of x is 3 x squared minus 1. So we will add 3 x squared

minus 1 and now we just need to simplify that. As you can see, we will get x cubed. When

we add 2 x squared and 3 x squared together, we will get 5 x squared and then of course

our negative 1. And that’s it. That’s really as far as we can simplify. We’ve

also been asked to give the domain of each of these combinations of functions. So we

will go ahead and do that and we will say that the domain here, the domain is equal

to – the domain of any polynomial function like this is all real numbers so we can just

denote that with all real numbers like that. As far as the difference of the two functions,

really similar, we have f minus g of x here. So we will just take f, just x cubed plus

2 x squared and we will subtract g of x. Don’t forget to put g of x in parenthesis like that

so that you make sure you distribute the negative sign. So when we subtract here, we will get x cubed.

2 x squared minus 3 x squared will give us a negative x squared and then we have minus

negative 1 which will give us a plus 1. Again, we’ve really simplified as much as we can.

So now we just need to indicate the domain and in this case, again we have a polynomial

function so the domain is all real numbers. If we look here at the product of these two

functions f times g of x, similar concept, we will just be multiplying the two together

so x cubed plus 2 x squared and then multiply that by g of x which is 3 x squared minus

1. So now we will be expanding these with our FOIL method. So we will have x cubed times

3 x squared which will give us 3 x to the fifth. x cubed times a negative 1 will give

us negative x cubed. 2 x squared times 3 x squared will give us plus 6 x to the fourth

and 2 x squared times the negative 1 will give us a negative 2 x squared. So now when we reorder our terms, we will

get 3 x to the fifth plus 6 x to the fourth minus x cubed minus 2 x squared and again

we have a polynomial which means that our domain is all real numbers. Finally our last function here, the quotient

of f and g, we will take f of x and we will divide it by g of x. So we will see that our

numerator here will be f of x which is x cubed plus 2 x squared and we will divide that by

g of x which we know to be 3 x squared minus 1 and there’s really not a whole lot we

can do here to simplify. We could factor out an x squared from our numerator but that wouldn’t

allow us to cancel anything. So this will be our quotient function. We just need to

figure out what the domain is. Remember that the domain of a rational function

like this, rational function being the quotient of two polynomials, a polynomial and a numerator

and a polynomial and a denominator, is undefined wherever the denominator is equal to zero.

So the domain is going to be all real numbers except where the denominator equals zero. So we need to figure out where that is the

case. So we will say over here, 3 x squared minus 1 equal to zero, and we need to solve

this to figure out which part of our domain is undefined. So we will add 1 to both sides to get 3 x

squared equals 1. Divide both sides by 3 to get x squared equals one-third and then we

want to take the square root of both sides to solve for x. So we will get x equals the

square root of one-third. So we will say the square root of one-third

and remember that when we’re taking the square root of a fraction like this, of a

quotient, we can take the square root of the numerator and the denominator separately.

So we will get x equals the square root of 1 over the square root of 3. We know that

the square root of 1 is just 1 and then we have the square root of 3 here in the denominator. Remember we have to include a positive or

negative because we could get positive 1 over the square root of 3 or negative 1 over the

square root of 3. Both of those would make our denominator equal to zero and therefore

our quotient undefined. So we can say that the domain of this function

is equal to all real numbers except x equals positive or negative 1 over the square root

of 3. So we can just say the domain is x not equal to positive or negative 1 over root

3 and that indicates that x can be any value except these two and defines our domain. That’s

it. That’s how you find the combinations of functions and their domains. So I hope you found that video helpful. If

you did, like this video down below and subscribe to be notified of future videos.

1

## 49 Comments

## sawen sh

you're the best

## ThePinoyMamba

Keep on posting. It really helps

## Cameron Cole

I love the way you present your information. It helps a lot!

## Krista King

awesome!! π

## Ben Huisman

Thank you so much!!! I will be getting an A on my math test tomorrow. Hopefully.

## Krista King

I hope so too!! π

## tigersport17

I can actually pay attention to you because you're hot. I'm actually motivated to learn math

## Felipe Blanco

im so getting an A tomorrow! All thanks to this video! π

## Krista King

Hope you rocked it! π

## Tim Hudyma

Thank you, I have a test on this tomorrow too, this helped a lot.

## Krista King

you're welcome, i'm so glad!! good luck on your test!! π

## alex lagunas

Thank you!!

## Krista King

I'm so glad! You're welcome! π

## Krista King

I attended the University of Notre Dame, but I'm not a teacher. I've tutored for many years which got me into making the videos. π

## chifun86

Can I be your friend?

## Krista King

Consider it done! π

## muhammad nurharith

this might sound stupid but could you help me with g/f(x)

f(x)=square root of x-1

g(x)= x+5

and teach me the domain also.

## Rick Rod

thank you so much for this helpful video !!!

## Krista King

You're so welcome!

## Robert Olsen

I always thought that you can't have a square root in your domain.

## Maria M

Thank you!!! Very helpful π

## Krista King

You're welcome, I'm so glad!! π

## Krista King

you just can't put a negative value underneath a square root, so values that result in a negative number under the square root can't be included in the domain. π

## Robert Olsen

I'm sorry, I meant the denominator. I thought you couldn't have a square root in the denominator….

## Krista King

oh, that makes more sense. π yes, very often we rationalize the denominator to get the square root out of there, but i still commonly see it left as-is, so it's up to you. if you want to be safe, just rationalize it. π

## LollyGagers-Meechie

Thank Yu So Much U Saved My Life !

## Steve Cucumber

Very good video and you're not bad at all.:)

## Bhoopinder Masawan

great video, great maths thanks

Bhupi

## Khalil Shadeed

well done. helpful

## 315catlopez

Thank you so much! I now understand what I'm learning for my class π

## R Nascimento

How can I proceed in this case below?

1. f(x) . f(y) = f(x + y)

2. f(1)= 2

3. f( V2) = 4

So calculate f(3+V2).Β

## EchoFour Arch

Great job! only complaint, the closed captioning blocks out bottom of board.. Still awesome though!

## hamdard zabuli

you are the moust good teacher .and very pretty .i learn more from your lession .long life for you .

## Jorge Navarrete

I still don't really understand how the domain is AR#

## Ali Kat

Why don't you rationalize the denominator for the domain of f/g?

## Ashley Victoria

Thanks for the help~! ^_^

## Isah Liman

Youre Amazing.. Thank You For Helping Us

## ejboy111

Thanks sooo much! This is soo much easier than what it looks and you teach really well.

## fady Alfons

β€οΈβ€οΈβ€οΈ

## Alpha24985

.The domain of the function should by determined before simplifying the function. for example let f(x)=x^2-1 & g(x)=x-1and we need to identify the domain of (f/g)(x).after simplifying, (f/g)(x)=x+1 and its domain =R but this is not true because (f/g)(x)=(x^2-1)/(x-1) and its domain = R-{1} not R

## Omega Edits

I'm a junior in high school and my math teacher is awful. Thank you for explaining what she never does!π€

## aliissr

very helpful thanxxxx π

## Majid Rana

nice and gooD

## botshelo popie komane

well done

## Parham Gousheh

Their domain is the same solve some without the same domain ty

## Victoria5653

Thank you. Short, sweet, and to the point. Just what I needed.

## Salim Ali

I m from india thanks for helping.

## Shawn Fowles

Find (f/g)(x) and (g/f)(x) for the functions given by f(x) = radical x and g(x) = radical 4-x^2. Then find the domains of f/g and g/f. That is a problem from my precalculus textbook. If you can do a video on how you would solve this I would really appreciate it.

## chase miles

will you marry me