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Worked example: Evaluating functions from equation

Learn how to evaluate f(x)=49-x² at x=5. Created by Sal Khan.

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  • duskpin seedling style avatar for user nesla-anguh
    Can anybody help me find the output, k, when the input, t, is −7.
    k = 10t-19
    k = ?

    I need a hand. Thank you
    (15 votes)
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  • aqualine ultimate style avatar for user ShevrillHD
    1. Can a function have name like add(x) instead of f(x)?
    2. Can a function have more than 1 argument, like f(a, b, c)?
    3. Can a function's argument have a name like "apes", "apples" instead of "x", "y"?
    (7 votes)
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    • purple pi pink style avatar for user ZeroFK
      1. Yes. In mathematics it is more common to use a single letter (sometimes a Greek letter), but a function name can be anything. After all it's just a way to communicate to other humans what you're talking about, changing a name doesn't change the math.
      2. Yes. A simple example is f(x,y) = x * y.
      3. Yes. See answer 1: single letter names are conventional, but not required. They're all just names. You could even draw pictures instead of using letters - in fact, Khan Academy does that in some early math videos.
      (11 votes)
  • blobby green style avatar for user 4300414835
    How do you know where the output is?
    (8 votes)
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  • duskpin ultimate style avatar for user Sophia
    Can Someone Help Me Solve This Function?
    f(0)^2
    where f(x)=3x+4
    (6 votes)
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  • blobby green style avatar for user Ralph Reznik
    What are the general rules for solving functions?
    (3 votes)
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  • hopper cool style avatar for user KDP28
    What Does the problem mean by "Is defined as"?
    (3 votes)
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  • blobby green style avatar for user corazonsanjose3
    how will you make a mathematical model for this:
    You and your classmates want to earn extra income this summer. To do
    this, you will sell Fun Shirts. The wholesaler has the following
    conditions: If you purchase 100 shirts, each shirt costs P120; if you
    purchase between 200 to 300 shirts, the price per shirt is lowered to
    P110; if you get more than 300 shirts, the price per shirt is lowered by 15. thank you.
    (3 votes)
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  • aqualine ultimate style avatar for user brycemc1
    Wouldn't the answer be 25 cause a neg. times a neg is 25?
    (2 votes)
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  • duskpin ultimate style avatar for user natalie.sch
    So the "f" in f(x) stands for function? As in solve the function with whatever the x variable may be?
    (2 votes)
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  • piceratops ultimate style avatar for user aaa
    how i do this plsz help me sombody
    (2 votes)
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Video transcript

The function f of x is defined as f of x is equal to 49 minus x squared. Find the value of f of 5. So whenever you're dealing with a function, you take your input. In this case, our input is going to be our 5. We input it into our little function box, and we need to get our output. And they defined the function box here as whatever your input is, take that, square it, and then subtract it from 49. So f of 5, every time I see an x here, since f of x is equal to this, every time I see an x, I would replace it with the input. So f of 5 is going to be equal to 49 minus-- instead of writing x squared, I would write 5 squared. So this is equal to 49 minus 25. And 49 minus 25 is equal to 24. And we are done.