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### Course: Get ready for Algebra 1 > Unit 4

Lesson 3: Functions intro- 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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# Worked example: Evaluating functions from graph

Evaluating a function at x=-1 using the graph of that function. Created by Sal Khan.

## Want to join the conversation?

- Why is it always f(x) and not any other letters?(12 votes)
- Wait until you get to Algebra 2 when you have to start combining multiple functions, you will start seeing g(x), h(x) etc. In Algebra I, you are just getting used to functional notation, but the power of functional notation over y= form will come later.(19 votes)

- I don't understand something. When asked to find f(x), what am I being asked to find? Where do I look to find it?(6 votes)
- When it says 'f(x)' it is generally talking about the y. So when you write an equation like f(x)=2x+3. it in other words is saying y=2x+3.
*I hope that made some sort of sense... If you want me to explain it in greater detail.. just let me know! <3*(18 votes)

- How to graph a Parabola(5 votes)
- A Parabola could be graphed when given the following skeleton equation: ax^2+bx+c. However, it is not easy to explain how to graph parabolas over comments, so it would be much wiser to follow MyAnchorHolds' suggestion and view the videos on Khan Academy.(5 votes)

- What if the function is f(-3) but there’s two points going vertically thru -3(7 votes)
- Then it is not a function. A function can only have one y value for every x value. Important to remember there can be multiple x values for a single y value. Kind of confusing but important to remember. if you know it, the vertical line test will tell you if something is a function.(7 votes)

- can anyone please tell me what would be the answer of

f : x > x^2 -5x

a) find the value of f(-2)

b) find the value of ff(2)

c) find the range of f if domain is {-1, 0, 1}

it would be really grateful of you(7 votes)- f(x)> x^2 - 5x so by substitution,

a) f(-2) > (-2)^2 - 5(-2) 4 - (- 10) = 14

f(-2)> 14

b) f(2) > (2)^2- 5(2) 4-10 = -6

f(2) > -6

c) f(-1) > (-1)^2 -5(-1) f(-1)> 6

f(0) > 0^2 - 5(0 f(0) > 0

f(1) = (1)^2 -5(1) f(1) > -4

I have not seen this type of problem with limited domain with inequality, but

since the range is the f(x),

my best guess on how to write it would be {f(x)>6, f(x)>0, f(x)>-4}(6 votes)

- I do not get this at all I have a test in a few days, this is SO complicated. I need some help. PLEASE HELP ME

And please answer fast(6 votes)- I know this isn't really a fast answer, but,

basically he is just saying that when the x coordinate is at -1, the y coordinate is 6 on the line.

He just wants you to find out the y coordinate by using the line, when x (F) is given to you.

Reply if you actually read this. you probably already got it, but ill write this anyway.(3 votes)

- Can I write the function as f=x+1?(4 votes)
- My answer: kinda. You can write it like that, and it is technically correct, but it is common practice to either write it in: y=mx+b or f(x)=mx+b.

Hope that helps!(5 votes)

- Sal Khan is smart, so why is the line all spaghetti like.(5 votes)
- This is hand drawn, so this is not that perfect though he's smart.(3 votes)

- why is the line in such a weird shape(4 votes)
- Likely because Sal wanted to show you a non-typical function.(3 votes)

- I am just trying to learn how to turn a graph into a function. That's all. So why didn't they explain dis better? HELP ME! Can someone tell me how to turn a graph into a function?(3 votes)
- You need to provide more info...

1) Are you asking how you determine if the graph is/isn't a function? The graph either is or isn't a function already. Use the vertical line test to determine if the graph is a function. If you can draw a vertical line anywhere in the coordinate plane and the line only crosses the graph once, then the graph is a function.

2) Or, are you being asked to create the equation that would create the graph and then show the equation in function notation? If this is what you were asked to do, you need to know what type of graph you have: linear, quadratic, polynomial, exponential, etc. Each have different techniques for creating an equation from the graph. Once you know the type of graph, you can use the search bar to find lessons on writing equations of that type.(4 votes)

## Video transcript

The function f of x is graphed. Find f of negative 1. So this graph right
over here is essentially a definition of our function. It tells us, given the allowed
inputs into our function, what would the function output? So here, they're
saying, look, what gets output when we input
x is equal to negative 1? So x equals negative 1
is right over here. x is equal to negative 1. And our function
graph is right at 6 when f is equal to negative 1. So we can say that f of
negative 1 is equal to 6. Let me write that over here.
f of negative 1 is equal to 6.