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### Course: 8th grade > Unit 3

Lesson 8: Functions- What is a function?
- Worked example: Evaluating functions from equation
- Function notation example
- Evaluate functions
- Worked example: Evaluating functions from graph
- Evaluate functions from their graph
- Equations vs. functions
- Manipulating formulas: temperature
- Obtaining a function from an equation
- Function rules from equations

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# Obtaining a function from an equation

Equations and functions are not the same thing, but they can be related in several ways. In this video, we obtain a function from an equation. The function represents the same relationship between the quantities in the equation.

## Want to join the conversation?

- Hi, there is this one problem in the exercise "Write function rules from equations" that I am having a hard time with. It wants me to enter the greek letter upsilon as part of the answer, but I don't know how. How can I do that??(86 votes)
- The input is b, so, f(b), the output is a, so f(b)=a, whatever input b we plug into our function, it's gonna output a.

Therefore, to satisfy the equation we need to solve the equation in terms of a, and then just replace the a in f(b)=a, and that's our function, bellow is a summary of the steps.

f(b)=a // whatever b we input, the function outputs a

4a+7b = -52 // this is the equation our a has to satisfy

a = -13-(7/4)*b // therefore we solve for a, so the equation stands true

f(b) = -13-(7/4)*b // we replace the a in f(b)=a with whatever a equals to(19 votes)

- I am doing the exercises now but I haven't gotten a single one right. I don't think Sal gave an adequate enough explanation. He only did a couple examples and when I look at the hints they seem lazy and not very in depth... XD what do I do?(36 votes)
- So here is one of the practice problems:

7m+2=6n−5 what is f(m)?

Since the independent variable is m indicated by being in the functional parenetheses, we must solve for the dependent variable, and the only one left is n, so lets solve for n

6n - 5 = 7m + 2 just flipped sides of equation

6n = 7m + 2 + 5 moved the -5 by adding 5 on both sides

6n = 7m + 7 combined like terms

n = 7/6 m + 7/6 divided all terms by 6

f(m) = 7/6m +7/6 substituted functional notation with equation notation

Does this help?(57 votes)

- Why would you solve for A if you are trying to figure out a formula for B ? In other words trying to get B by it self. For this problem I subtracted 4a by itself and -52 which then simplified to 7b= -52-4a / 7 which simplified even more to which b = -52-4a / 7(19 votes)
- You solve for a because a is the output or in other words a is the f(B). I think this is right. Any other ideas?(9 votes)

- So basically -13 - (7/4b) is the same as a, which is the same as f(b). Am I right?(11 votes)
- Yes, you are correct.

When f(b) is given a "b" value we get an "a" value which is the same as "f(b)".

For example if we're given the "b" value of 0, then f(0) = -13 and that is what "a" would be if "b" is zero. "f(b)" is a more concise way to say this since it says what value of "b" was used to get the "f(b)" value - to get a particular "a" value.

I hope this helps.(16 votes)

- In school, I have a teacher that doesn't explain anything. I am a 6th grader so in Algebra 1 and I am having a lot of trouble with functions. I can solve functions but creating a linear function equation is difficult for me. Could anyone help explain it to me in an easy way that I could understand?(16 votes)
- Me too! I don´t understand this lesson. 6th grader(1 vote)

- I was starting to understand this until I ran into a problem on the exercises.

a - 7 = 3(b+2)

I was able to get a by itself

a = 3(b+2)+7

in the hints it said it would simplify down to

a = 3b + 13.

My question is how do you get 13?(0 votes)- Look at the second line:

a=3(b+2)+7 ---> a=3*b+3*2+7 ---> a=3b+6+7 ----> a=3b+13

That is how you get 13!

Greetings,

Pedro(21 votes)

- this topic definitely needs more explanation !!(8 votes)
- The first concept to know is the f(x) and Y refer to the same thing. F(x) is Y. So, if you have an equation that you want to change to function notation, you:

1) Solve the equation for Y

2) Then replace Y with f(x)

That's all it takes.(5 votes)

- this is really hard(8 votes)
- By now, you should have experience solving equations for "y" to change a linear equation into slope-intercept form. Use those skills.

The 1st step is to solve the equation for "y" (isolate "y" by itself on one side of the equation.

The 2nd step is to replace "y" with f(x), or whatever name you were asked to use for your function.

Hope this helps.(1 vote)

- so technically an equation in two variables is a function and equation in one variable is just an equation(3 votes)
- Not necessarily.

A function is a relationship where each input value (X) will create only one output value (Y). Basically, a single input value, can't create 2 different output values.

Any equation with one or two variables that meet this definition would be a function.

y = 5 is the equation for a horizontal line. It is a function where all values of X have a y-value = 5. Yet it has one variable.

x = 5 is the equation for a vertical line. It is not a function because in this situation, the input value (x=5) has an infinite number of output values.

All other equations of lines (Ax + By = C) are functions because the meet the definition of a function.

But, the equation of a circle, which also uses 2 variables, would not be a function because each input value (x) creates 2 output values (y).

If an equation is not a function, sometimes we can still change it into a function if we restrict its inputs or outputs.

Hope this makes sense.(10 votes)

- What? why are we solving for a? why not b? where is the input for the function here? what does solving for a have to do with this f(b). How is f outputting a? im so confused(3 votes)
- If the problem is asking you to create function f(b), then it is telling you that the input value is defined by "b", not "a". So, "a" becomes the variable that you isolate and switch to f(b).

If you solved for "b", then the input value would need to be "a" and the problem didn't ask you to create f(a).

Hope this helps.(10 votes)

## Video transcript

For a given input value b, the function f outputs a value a to satisfy the
following equation 4a plus 7b is equal to negative 52. So for a given input b, the function f, the function f
will output an a that satisfies this
relationship right over here for the a and the b. Write a formula for f of b in terms of b. So we want to do,
we just want to solve for -- if we're given a "b", what "a" does that
imply that we have to output? Or another way to think about it is
-- let's just solve for a, or we could think about a as being a
function of b. So let's write this. So we have 4a plus 7b -- is equal to negative 52. So I can solve for a in terms of b, that any b that I have --
Let's say these b's are on the right hand side I can put it in. I can substitute that value
for b and I can just solve for a. I can solve for a that needs to be
outputted. So let's do that. Let's solve for a.
So I want to get all the a on I wanna just have an a leftover on the
left hand side, and have everything else on the right hand side including the b's. So let's get rid of this b on the left hand
side. And I can do that by subtracting 7b.
Of course I wanna do that to both sides. I can't just do an equation and do an operation only on one side like
that. So let's subtract, and we are left with
we are left with -- the 7b's add up to zero. 7b minus 7b.
We're left with 4a is equal to negative 52 minus 7b, minus 7b. Now, to isolate the a here,
just to have an a here instead of 4a, we can divide both sides by 4. We can
divide both sides by 4. So I'm gonna divide everything by 4. And on the left hand side, we got
our goal. We are left with an a is equal to -- Now
what's negative 54 divided by -- What is the negative 52 divided by 4?
So let's think about it. 52 is 40 plus 12. 40 divided by
4 is 10. 12 divided by 4 is 3. So it's gonna be 13. Negative 13.
So it's negative 13 minus 7/4 b minus 7/4 b. So given a "b", if you give me a "b", I can put
that value right over here, and I can calculate what the
corresponding a needs to be in order to satisfy this relationship. So if I want a formula for f of b in
terms of b, I can say, look, you give me a "b", the output of our
function, which is f of a. The output of our function is
going to be -- is going to be negative 13 minus 7/4 b, because the output of our
function needs to be an "a" that will satisfy -- that will satisfy this equation up here. So hopefully that helped.