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anonymous

  • one year ago

Cubic help

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  1. anonymous
    • one year ago
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    Graph doesn't look very cubic to me though which makes me thing I did it wrong.

  2. anonymous
    • one year ago
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    what's the exact question?

  3. anonymous
    • one year ago
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    as in what is the exact wording of the question you're trying to answer. positive rate of change → function is increasing negative rate of change → function is decreasing

  4. mathmate
    • one year ago
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    Is it possible to post an image of the original question? That's the fastest way to get help!

  5. anonymous
    • one year ago
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    You're giving a lot of information, but there's no context

  6. anonymous
    • one year ago
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    So I can do cubic regression or do l*w*h where (12-x) represents the sides and (x) represents the height of the boxes... whichever one you guys are able to help me with.

  7. anonymous
    • one year ago
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    Kinda wanted to know how to do it by hand, but maybe that isn't good to know as a pre-algebra student.

  8. mathmate
    • one year ago
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    Hints: "where (12-x) represents the sides and (x) represents the height of the box)" means: So if the height (x)=1, then the sides are (12-1)=11, or 1*11*11, V=121 If height is 2, sides are 12-2=10, volume = 2*10*10=200 in general, if the height equals x, the volume is x(12-x)(12-x), which is evidently a cubic. Do you agree with the above calculations?

  9. anonymous
    • one year ago
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    Yeah that makes sense.

  10. mathmate
    • one year ago
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    Yes, a cubic is a polynomial, which is any expression in the form of "the sum of positive integer powers of a variable, each multiplied by a constant".

  11. mathmate
    • one year ago
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    I do not know how you got y=-12x^2 + 49.6x + 67.2 but the volume itself is already a cubic (third degree polynomial), and will not need to have a cubic regression, because it is exactly a cubic polynomial.

  12. mathmate
    • one year ago
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    But your table does not correspond to the given constraints of x=height, (12-x)=sides, so it cannot be used to "generate" your regression cubic.

  13. mathmate
    • one year ago
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    But your table does not correspond to the given constraints of x=height, (12-x)=sides, so it cannot be used to "generate" your regression cubic that represent the results of the problem.

  14. anonymous
    • one year ago
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    But if it's wrong then I will just go with the way you showed me.

  15. mathmate
    • one year ago
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    In your table, I do not understand what you meant by "size of corner cut ". I use x=height, as per instructions.

  16. anonymous
    • one year ago
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    size of corner cut meants (remember I used grid paper to make cubes) I had to cut off one square on each corner. Let me draw it for you okay. I'm really sorry for confusing you. I did a very bad job explaining everything.

  17. anonymous
    • one year ago
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    Give me a few minutes. please.

  18. mathmate
    • one year ago
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    So that means the height. |dw:1440381788262:dw|

  19. anonymous
    • one year ago
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    right but in the first drawing it's 10 by 10

  20. mathmate
    • one year ago
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    So you need to make a table: |dw:1440381900195:dw|

  21. mathmate
    • one year ago
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    Can you please look at my table and see if you agree with it?

  22. anonymous
    • one year ago
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    for that table you draw for 3 would be 12-3 = 9 3*9*9 or 243

  23. mathmate
    • one year ago
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    exactly!

  24. anonymous
    • one year ago
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    Yeah that makes sense to me.

  25. anonymous
    • one year ago
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    So was my equation wrong then? I can't get an equation from your table you drew right?

  26. mathmate
    • one year ago
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    So use this new table and try to fit a cubic and see what you get. "Our" volume is x(x-2)^2=x^3-24x^2+144x. Your "regression" cubic should give exactly the same expression.

  27. mathmate
    • one year ago
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    Remember to fit a cubic you need at least 4 points, and preferably more if you're doing regression.

  28. anonymous
    • one year ago
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    wait what do I put in for x?

  29. anonymous
    • one year ago
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    Are you there @mathmate

  30. mathmate
    • one year ago
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    Sorry, I was afk. a bunch of numbers, like 1,2,3,4,5,6, or 2,4,6,8,10, ... If everything is calculated correctly, they should all give the same cubic polynomial.

  31. mathmate
    • one year ago
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    The numbers don't have to be evenly spaced like my examples, they could be x=1,2,5,10,21 if you want.

  32. mathmate
    • one year ago
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    Oops, cannot be greater than 12! So forget the 21!

  33. anonymous
    • one year ago
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    I graphed this on my paper I did my y axis by 10's and my x axis 12345 but skipped every two lines

  34. anonymous
    • one year ago
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    not sure if that's relevant

  35. mathmate
    • one year ago
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    Trick to graphing by hand is to get the smallest and largest y to start. So start with the table. |dw:1440383506458:dw| so make a y-scale high enough for 256, and x from 1 to 10.

  36. anonymous
    • one year ago
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    okay.

  37. anonymous
    • one year ago
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    okay thanks for your help. Really appreciate you sitting here and going over this with me :)))

  38. mathmate
    • one year ago
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    No problem! :)

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