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Inverse equations

  1. Apr 13, 2008 #1
    Hello,
    I have a quick question for you guys.


    What is it called when you switch the signs of the equation to their opposite?

    For example, does 10 + 6 = 16 invert to 10 - 6 = 4 Is this additive inverse?


    Or, does 14 x 7 = 98 invert to 14 / 7 = 2 Multiplicative inverse?

    Thanks in advance.
     
  2. jcsd
  3. Apr 13, 2008 #2

    lurflurf

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    in short yes

    if x*y=1 x is the multiplicatie inverse of y (also y is the multiplicatie inverse of x)
    if x+y=0 x is the additive inverse of y (also y is the additive inverse of x)

    Those are the nice names
    bad people disrespect these numbers by calling them insults like opposite and reciprical
     
  4. Apr 13, 2008 #3

    lurflurf

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    wow
    no
    10 + 6 = 16
    becomes
    -10 +(-6) = -16

    14 x 7 = 98
    becomes
    (1/14)*(1/7)=1/98

    but what you likely want is
    10 + 6 = 16
    so
    10=16-6 or 6=16-19
    and
    14 x 7 = 98
    so
    7=98/14 or 14=98/7

    This is often used with variables like
    10+x=16
    so
    x=16-10
    x=6

    14 y = 98
    so
    y=98/14
    y=7
     
  5. Apr 15, 2008 #4

    So, when you invert a regular equation. x + y = z the answer will always be -z

    But when you invert a multiplication/division equation x * y = z the answer will always the the reciprical of z.


    What is it called when you switch - to / and + to * ?
     
  6. Apr 15, 2008 #5
    Oh, silly me, I thought this was about inverse functions. I should pay more attention.
     
  7. Apr 16, 2008 #6

    HallsofIvy

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    Staff Emeritus
    Science Advisor

    Perhaps it would help if you explained what in the world you are talking about! You started talking about "inverting" equations, now you are talking about inverting operations.

    Yes, the "inverse" or "opposite" of addition is subtraction and the "inverse" of multiplication is division. Those apply only to specific operations, not to entire equations. I have no idea what you mean by "invert a regular equation".

    If I wanted to solve x+ y= z for x, then I do the "inverse" of "add y" (since in the equation y is added to x) which is "subtract y". Subtracting y from both sides (whatever you do to one side of an equation you must do to the other) and get x+ y- y= z- y or x= z- y.
     
  8. Apr 25, 2008 #7
    HallsofIvy, I'm curious more about inverting operations, and why:

    +1 - +1 = 0 and -1 + -1 = -2 on my calculators.
     
  9. Apr 25, 2008 #8

    HallsofIvy

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    Science Advisor

    Well good- then your calculator is working properly! "Subtraction" is technically defined as "adding the additive inverse". To your calculator (1)- (1) means 1+ (-1) which is, by definition of "additive inverse", 0. -1+ (-1) means you are adding two additive inverses, Since (1)+ (1)= 0, and addition satisfies both the "associative" and "commutative" laws,
    (1+ 1)+ ((-1)+ (-1))= (1+ (-1))+ (1+ (-1))= 0+ 0= 0. That means that ((-1)+ (-1)) is the additive inverse of 1+ 1= 2. That is, (-1)+ (-1)= -2.
     
    Last edited: Apr 25, 2008
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