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Number divided by something divided by something?

  1. Nov 19, 2017 #1
    1. The problem statement, all variables and given/known data

    For example:
    400x/(x/y)

    Can also be written as:

    400x
    ____
    x
    ____
    y


    So now, the answer will be 400y, but my question is, how does “y” get to be multiplied by 400? (Why isn’t it 400 divided by “y”?) How would I undo it for it to not be in the numerator?

     
  2. jcsd
  3. Nov 19, 2017 #2

    David Lewis

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    Gold Member

    When the denominator gets smaller, the quotient gets larger.
    As y increases in size, the denominator gets smaller.
     
  4. Nov 19, 2017 #3
    Hmm... not what I was asking for. Let me see if I can rephrase it.

    Here’s an ex of what I’m trying to say: https://imgur.com/a/WhnOo
     
  5. Nov 19, 2017 #4

    jedishrfu

    Staff: Mentor

    You need to pay attention to the parentheses.

    You have 400x divided by (x/y) so the x’s cancel leaving 400 / (1/y) which is the same as 400 * y

    How do you get that answer by multiplying top and bottom by y.
    Code (Text):

    400 * y      400*y
    ————————— = ————-
    (1/y) * y      1
     
     
  6. Nov 19, 2017 #5

    David Lewis

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    Gold Member

    It's 4000J / 100J / 1oC

    So in the denominator, we have 100J / 1oC (=100)
     
  7. Nov 19, 2017 #6

    jedishrfu

    Staff: Mentor

    You changed the problem by removing the parentheses to be (4000j / 100j) / 1c

    I think the 100j per 1c is more of a unit of measure so that your answer will be in degrees c.
     
  8. Nov 19, 2017 #7

    jedishrfu

    Staff: Mentor

    An analogous problem would be I travel for 100 kilometers at 25 km/hr hence it takes me 4 hours to make the journey.
     
    Last edited: Nov 19, 2017
  9. Nov 19, 2017 #8

    Mark44

    Staff: Mentor

    Because dividing by a/b is defined to be the same as (i.e., equal to) multiplication by the reciprical of a/b. In other words, for your problem,
    ##\frac {400x}{\frac x y} = 400x \cdot \frac y x = \frac {400x y} x = 400y##
     
  10. Nov 19, 2017 #9
    I see, thank you.
    I don’t think I was ever really shown that in school, so I never thought about it that way. Is there a guide for practice/example of how this works?
     
  11. Nov 19, 2017 #10
    Also, so this means if a problem is x/y/z , you cannot do (x/y)/z, right?
     
  12. Nov 19, 2017 #11
    Oh I remember, it’s that one thing I learned from fifth grade I think. It’s when you divide a fraction by a fraction, it’s the same as multiplying the first fraction by the reciprocal of the second.
    So in this case, if it’s 400x/x/y, we could put it as (400x/1) / (x/y), and rewrite it as (400x/1)*(y/x). Am I write about this?
     
  13. Nov 20, 2017 #12

    Mark44

    Staff: Mentor

    Yes, you can do this. By the usual order of operations, and since both operations are division, you can do the first division, and then divide that by z.

    So x/y/z is the same as
    $$\frac{\frac x y}{z} = \frac x y \cdot \frac 1 z = \frac {x y}z$$

    Edit: The last expression should be ##\frac x {yz}##

    Yes, that's right.
    Yes, you are right. Note that "write' is an action you do with a pen or pencil.
     
    Last edited: Nov 20, 2017
  14. Nov 20, 2017 #13

    Ray Vickson

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    Science Advisor
    Homework Helper

    Using LaTeX makes it clearer: your original expression ##400\, x /(x/y)## can be displayed as
    $$\frac{400\, x}{ \displaystyle \frac{x}{y}} $$
    This can also be written as
    $$\frac{400 \,x}{x} \times \frac{1}{\displaystyle \frac{1}{y}} $$
    or more clearly as
    $$\frac{400 \,x}{x} \times \frac{1}{\displaystyle \left(\frac{1}{y}\right)} ,$$
    and
    $$\frac{1}{\displaystyle \left(\frac{1}{y}\right)} = y$$
     
  15. Nov 20, 2017 #14

    Ray Vickson

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    Science Advisor
    Homework Helper

    **********************
    Oops:

    $$ \frac{x}{y} \cdot \frac{1}{z} = \frac{x}{yz} \;\;!$$

    The Classic Maple interface accepts "a/b/c" instead of "a/(b*c)"

    ***********************

     
    Last edited by a moderator: Nov 20, 2017
  16. Nov 20, 2017 #15

    Mark44

    Staff: Mentor

    Thanks for catching that, Ray. I have edited my earlier post.
    Are you saying that Classic Maple treats a/b/c as if it were written a/(b * c)?
    That's odd, since programming languages generally associate arithmetic operators at the same precedence from left to right, with a + b + c being interpreted as if written (a + b) + c, and a / b / c as if written (a /b) /c.
     
  17. Nov 20, 2017 #16

    Ray Vickson

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    Science Advisor
    Homework Helper

    Saying "a/b/c = a/(b*c)" is not odd at all; it is completely consistent with the usual priority rules, because (a/b)/c = a/(b*c).

    However, the new Maple interface does not do that, because it immediately converts a typed a/b to ##\frac a b##; in other words, it would not even let a user write "a/b" on screen, and so would not even allow one to type in a/b/c on-screen---it would convert fractions immediately in real time to
    $$\frac{a}{\frac{b}{c}}$$
    However, a/b/c copied from a classical worksheet and pasted into the new interface would appear on-screen as "a/b/c" and would still evaluate as ##\frac{a}{bc}##, just the way it did in the classic interface. To get a/(b*c) in the new interface, one types a/b*c, and to get (a/b)*c one types a/b→*c, where → is the right-arrow key. The → key closes out a fraction or a power, putting one back on the previous input level.


    BTW: the new interface can be quite handy when one get used to it: entering ##a/(b+c)## is easy: just type a/b+c, because it starts off as ##\frac{a}{b \;\;\;}##, then waits for more input from the user. If one does not leave the fraction (by prsssing the →) then when one types the "+c" that immediately goes to the right of the "b" in the denominator. If one actually wants ##\frac{a}{b}+c## it needs "a/b →+c", which can be a bit annoying.
     
  18. Nov 20, 2017 #17

    Mark44

    Staff: Mentor

    I misspoke. It's early in the morning for me, and my brain isn't operating at full capacity yet. I must have been thinking of something like a/b*c, where some might interpret this to mean ##\frac a {b \cdot c}## and others as ##\frac a b \cdot c##.

    The PEDMAS precedence rules aren't as clear as the precedence rules in programming languages, particularly C and languages derived from C. The programming precedence rules specify not only the precedence of all the operators, but also how the operators associate or group, for expressions that include multiple operators of of the same precedence level.

    A mathematical expression such as a/b*c is ambiguous, based on several threads posted here in the past, but the same expression is crystal clear if parsed by a C or C++ compiler. Since the division and multiplication operators are at the same precedence level, and associate left-to-right, a C compiler would interpret a/b*c as if written (a/b) * c.
     
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