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# Evaluating discrete functions

Given the graph of a discrete function, Sal shows how to evaluate the function for a few different values.

## Want to join the conversation?

• Which function passes through the points (2, 3) and (4, 4)
(6 votes)
• There may be a variety of functions that go through the points, so specify if you want a linear function, quadratic function, absolute value, or what.
(6 votes)
• whats a way we would use discrete function in real life.
(3 votes)
• You could write an "is a function of" statement and think about it. For example, "total cost" is a function of "*# of shoes*". Then you can plot it, and if you can't buy half of a shoe, then it's discrete. That, or you can't buy more than and x number of shoes.
(5 votes)
• f(-7) means: whats the y value when x is -7, right?
(3 votes)
• you said that y=h(x) is not a function if the graph has 2 y values.
what if the function was x=h(y)
would it be the other way around? would having 2 x values make it no longer a function?
(1 vote)
• The definition of a function is as follows:
A function takes any input within its domain, and maps this to 1 output.
The domain of a function is what input values it can take on. For an example, the function f(x)=1/x cannot take on x values of x=0 because that would make the function undefined (1/0 = undefined).
The range is what possible y values a function can take on. Some functions, such as 'y=x' have a infinite range, meaning they can output any y value. Other functions, such as 'y=sin(x)' have a run from -1 to 1.

Function Notation
It is very important to understand the notation we use for functions.
An equation is simply two expressions set to one another, like y=2x.
But if we were drawing the graphs of some of our equations then, then we might get confused. If we had two equations y=2x and y=5x-3 how could we differentiate between the two; aka reference one easily?

Well if we define a function as an equation then we can reference the equation as the function name.
The function name can be anything, f, g, h, etc.
We would write that like so:
f(x) = 25x
So we write the function name followed by parenthesis, and inside the parenthesis we would write our (independent) variables to that function. We can actually have more than 1 variable, which you'll see in advanced courses like Multivariable Calculus.
f(x, y) = 25x + 50y

Now we can easily reference as a function by its name like g(x) or h(x) which helps us differentiate between all of our functions.

Hope this helps,
- Convenient Colleague
(5 votes)
• what about the numbers it does not have a dot for
(2 votes)
• Well, I suppose you would find an equation (most likely equations, may find by math or graphing calculator), then you would solve for them.
You may use many methods for finding an equation for a scatter plot. You would find the best correlation and then find two points and use point-slope form and find the equations. But for scatter, plots that are supposed to function the x-values need to have different y-values to be considered a function.
And those are more advanced stuff you are asking about right now. Depending on what math level you are on, you either learned this before or are going to learn this in the future.
Tell me if this was confusing.
(3 votes)
• what if two different inputs have the same ouput. Is that a function?
(2 votes)
• That is okay, think of the quadratic parent function y=x^2, you have (-1,1)(0,0)(1,1), which is a function. So yes it is fine.
(3 votes)
• So Functions always have only 1 Y, right?
(2 votes)
• Most of the time, yes, though I am not sure. Please correct me if I am wrong!
(1 vote)
• why is the test so much harder than these videos ?
(1 vote)
• He wants us to understand it. If he just gives us the answer to making it easy, it's just memorization, not actual learning.
(2 votes)
• can someone help me with this problem?
h(46)=61.41+2.32(46). how would I solve this? I am getting stuck.
(1 vote)
• h(46)=61.41+2.32(46)
h(46)=61.41+(2.32 • 46)
h(46)=61.41+106.72
h(46)=168.13
Hope this helps (even though I'm late 4 months)
(2 votes)
• At , what is a unique value? And why it's not a function when you have two possible values?
(1 vote)
• The Definition of a function, in general, is the relationship between a set of inputs to another set of outputs, but there has to be one and only one output for one input. So, in the video, he just wanted to say that there is only one value when he said,"a unique value."
(2 votes)

## Video transcript

- [Instructor] What we have here is a visual depiction of a function. And this is a depiction of Y is equal to H of X. Now when a lot of people see function notation like this they see it as somewhat intimidating until you realize what it's saying. All a function is, is something that takes an input, in this case it's taking X as an input and then the function does something to it and then it spits out some other value which is going to be equal to Y. So for example what is H of four based on this graph that you see right over here? Pause this video and think through that. Well all H of four means is when I input four into my function H what Y am I spitting out? Or another way to think about it, when X is equal to four, what is Y equal to? Well when X is equal to four, my function spits out that Y is equal to three. We know that from this point right over here, so Y is equal to three, so H of four is equal to three. Let's do another example. What is H of zero? Pause the video try to work that through. Well all this is saying is if I input an X equals zero into the function what is going to be the corresponding Y? Well when X equals zero we see that Y is equal to four. So it's as simple as that. Given the input what is going to be the output? And that's what these points represent, each of these points represent a different output for a given input. Now it's always good to keep in mind one of the things that makes it a function is that for given X that you input you only get one Y. For example if we had two dots here, then all of a sudden or we have two dots for X equals six, now all of a sudden we have a problem figuring out what H of six would be equal to because it could be equal to one or it could be equal to three. So if we had this extra dot here, then this would no longer be a function. In order for it to be a function for any given X, it has to output a unique value, it can't output two possible values. Now the other way is possible. It is possible to have two different X's that output the same value. For example, if this was circled in what would H of negative four be? Well H of negative four when X is equal to negative four, you put that into our function it looks like the function would output two. So H of negative four would be equal to two. But H of two is also equal to we see very clearly there, when we input a two into the function, the corresponding Y value is two as well. So it's okay for two different X values to map to the same Y value that works. But if you had some type of an arrangement, some type of a relationship where for a given X value you had two different Y values, then that would no longer be a function. But the example they gave us is a function assuming I don't modify it.