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## Algebra (all content)

### Course: Algebra (all content)ย >ย Unit 7

Lesson 1: Evaluating functions- What is a function?
- Worked example: Evaluating functions from equation
- Evaluate functions
- Worked example: Evaluating functions from graph
- Evaluate functions from their graph
- Worked example: evaluating expressions with function notation
- Evaluate function expressions

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# Worked example: Evaluating functions from graph

CCSS.Math: , ,

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?(11 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.(8 votes)

- How to graph a Parabola(3 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.(2 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?(5 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*(8 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(6 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}(5 votes)

- What if the function is f(-3) but thereโs two points going vertically thru -3(4 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.(4 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(4 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.(2 votes)

- Can I write the function as f=x+1?(2 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!(3 votes)

- id love to ask a question, how am I SUPPOSED to know that the grid point of x=4 will make the y plot equal 1 when its NOT STATED?

I'm sorry, but I am quite frankly wondering how I'm supposed to know that the plot you wanted me to use is (4,1)?? you listed that 4=x, GREAT, but that Y value could be ANYTHING ON THE GRAPH, how am I supposed to know that I'm supposed to use (4,1)? how?(2 votes)- I think you don't understand. See, you're supposed to find 4 on the x-axis and then see where the line intersects that x value.(2 votes)

- why is it always fx(1 vote)
- Functions are not always f(x). It just happens to be the most frequently used. A function can be names by an letter. And, the input variable doesn't have to be "x" either. It is quite common if a function has a input that is time, to see the letter "t" used instead of "x".(4 votes)

- in a graph of y-f(x) what is the value of y=(0)(2 votes)
- Drawing a line of x=0 (as you are finding what is f(0)), the intersection point is at (0,
**8**), so 8 is your answer.

btw just remember that formatting matters and it's f(0) (function of 0) instead of y=0 cause they mean different things.

hopefully that helps !(2 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.