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Equations vs. functions

Equations and functions are not the same thing, but they can be related in several ways. Watch Jesse Roe and Sal talk about the difference between equations and functions. Created by Sal Khan.

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  • piceratops seed style avatar for user 1598281
    if f(x) equals y then why bother use f(x).
    (26 votes)
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    • leaf blue style avatar for user Stefen
      The following is from: http://www.purplemath.com/modules/fcnnot.htm
      For functions, the two notations mean the exact same thing, but "f(x)" gives you more flexibility and more information. You used to say "y = 2x + 3; solve for y when x = –1". Now you say "f(x) = 2x + 3; find f(–1)" (pronounced as "f-of-x is 2x plus three; find f-of-negative-one"). You do exactly the same thing in either case: you plug in –1 for x, multiply by 2, and then add the 3, simplifying to get a final value of +1.

      But function notation gives you greater flexibility than using just "y" for every formula. Your graphing calculator will list different functions as y1, y2, etc. In textbooks and when writing things out, we use names like f(x), g(x), h(x), s(t), etc. With this notation, you can now use more than one function at a time without confusing yourself or mixing up the formulas, wondering "Okay, which 'y' is this, anyway?" And the notation can be usefully explanatory: "A(r) = (pi)r2" indicates the area of a circle, while "C(r) = 2(pi)r" indicates the circumference. Both functions have the same plug-in variable (the "r"), but "A" reminds you that this is the formula for "area" and "C" reminds you that this is the formula for "circumference".
      (62 votes)
  • leafers seed style avatar for user pranjalagarwal19
    What is a vertical line test ?
    (18 votes)
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    • purple pi pink style avatar for user ZeroFK
      A test to determine whether a relation is a function. When you graph a function (as Sal did at ), draw a vertical line at every point on the X axis (of course that's not practically possible, since there are infinite points, and besides, the whole paper would be black with lines - but do it in your head). If none of those vertical lines crosses the graph at more than one point, the relation is a function.
      (34 votes)
  • starky ultimate style avatar for user ЛØ FЦЛ ΛŁŁØƜƐÐ
    arent linear equations technically functions?
    (9 votes)
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  • blobby green style avatar for user Ijaz Durrani
    Instead of beating about the bush --A function has at least 2 variables: an output variable and one or more input variables. An equation states that two expressions are equal, and it may involve any number of variables (none, one, or more). A function can often be written as an equation, but not every equation is a function.
    (13 votes)
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    • stelly blue style avatar for user Kim Seidel
      FYI: Horizontal lines like y = 5 or f(x)=5 are functions but you only see the one variable in the equation. Thus, your first sentence is a little misleading. A function is a set ordered pairs where each input creates exactly one output. It does not need to be an equation. The equation for the functions does not need to have 2 variables. It just needs to be able to have / create ordered pairs that satisfy the rules for a function.
      (14 votes)
  • blobby green style avatar for user Phoebe B
    when am I ever going to use this in my adulthood?
    (9 votes)
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  • blobby green style avatar for user singergirl71
    Tell me if this question makes any sense: I noticed that using a graph was very affective in explaning what a function is. Can creating a graph resembling the one that was used in the video be the best way to solve it?
    (14 votes)
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  • orange juice squid orange style avatar for user Marcus Loe
    What exactly is the difference between an equation and a function? As far as I could tell, Sal only gave examples of each and possible statements that could be made about equations or functions, but no clear definition as to what they are or the difference between them.
    (9 votes)
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  • blobby green style avatar for user Jen
    what is one benefit of using functions?
    (5 votes)
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    • leaf green style avatar for user Christofer
      Functions have very many benefits, because functions have so many uses. As you learn more advanced forms of mathematics, you will find that functions can be used to simplify a concept or a statement. For example, 2x + 3 = y
      One can say that a f(x), or a function of x, = y. So you can rewrite that equation as f(x) = 2x + 3. Now you can substitute "x" for any number you like. f(1) = 2(1) + 3. f(2) = 2(2) + 3. Hope this helped! Happy functioning!
      (7 votes)
  • marcimus orange style avatar for user Lysandre Ishvar
    Aren't all functions also equations since functions contain an = sign (e.g f(x)=x+2x). An answer would be appreciated and I wish everyone a great day.
    (5 votes)
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  • piceratops ultimate style avatar for user Cosmic Phantom
    I noticed complaints (some including insults) targeted at Jesse in this section, for his silence especially. The video is likely scripted and that means he might not be responsible for his silence in the video. Blaming him probably won't do much.
    (7 votes)
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Video transcript

SALMAN KHAN: I'm here with Jesse Roe of Summit Prep. What classes do you teach? JESSE ROE: I teach algebra, geometry, and algebra II. SALMAN KHAN: And now you're with us, luckily, for the summer, doing a whole bunch of stuff as a teaching fellow. JESSE ROE: Yeah, as a teaching fellow I've been helping with organizing and developing new content, mostly on the exercise side of the site. SALMAN KHAN: And the reason why we're doing this right now is you had some very interesting ideas or questions. JESSE ROE: Yeah, so as an algebra teacher, when I introduce that concept of algebra to students, I get a lot of questions. One of those questions is, what's the difference between an equation and a function? SALMAN KHAN: The difference between an equation verses a function, that's an interesting question. Let's pause it and let the viewers try to think about it a little bit. And then maybe we'll give a stab at it. JESSE ROE: Sounds great. So Sal, how would you answer this question? What's the difference between an equation and a function? SALMAN KHAN: Let me think about it a little bit. So let me think. I think there's probably equations that are not functions and functions that are not equations. And then there are probably things that are both. So let me think of it that way. So I'm going to draw-- if this is the world of equations right over here, so this is equations. And then over here is the world of functions. That's the world of functions. I do think there is some overlap. We'll think it through where the overlap is, the world of functions. So an equation that is not a function that's sitting out here, a simple one would be something like x plus 3 is equal to 10. I'm not explicitly talking about inputs and outputs or relationship between variables. I'm just stating an equivalence. The expression x plus 3 is equal to 10. So this, I think, traditionally would just be an equation, would not be a function. Functions essentially talk about relationships between variables. You get one or more input variables, and we'll give you only one output variable. I'll put value. And you can define a function. And I'll do that in a second. You could define a function as an equation, but you can define a function a whole bunch of ways. You can visually define a function, maybe as a graph-- so something like this. And maybe I actually mark off the values. So that's 1, 2, 3. Those are the potential x values. And then on the vertical axis, I show what the value of my function is going to be, literally my function of x. And maybe that is 1, 2, 3. And maybe this function is defined for all non-negative values. So this is 0 of x. And so let me just draw-- so this right over here, at least for what I've drawn so far, defines that function. I didn't even have to use an equal sign. If x is 2, at least the way I drew it, y is equal to 3. You give me that input. I gave you the value of only one output. So that would be a legitimate function definition. Another function definition would be very similar to what you do in a computer program, something like, let's say, that you input the day of the week. And if day is equal to Monday, maybe you output cereal. So that's what we're going to eat that day. And otherwise, you output meatloaf. So this would also be a function. We only have one output. For any one day of the week, we can only tell you cereal or meatloaf. There's no days where you are eating both cereal and meatloaf, which sounds repulsive. And then if I were to think about something that could be an equation or a function, I guess the way I think about it is an equation is something that could be used to define a function. So for example, we could say that y is equal to 4x minus 10. This is a potential definition for defining y as a function of x. You give me any value of x. Then I can find the corresponding value of y. So this is at least how I would think about it.