Results 1 to 3 of 3
Like Tree2Thanks
  • 1 Post By skeeter
  • 1 Post By Prove It

Math Help - how to explain?

  1. #1
    Junior Member
    Joined
    Aug 2011
    Posts
    60

    how to explain?

    how to explain -1 x -1 = +1 (minus x minus equals plus). Is there any practical example (explanation) for this? I have read as a mathematical formula, but how it is true? Kindly enlighten me. Any guidance on this issue is welcome.

    With advance thanks.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor
    skeeter's Avatar
    Joined
    Jun 2008
    From
    North Texas
    Posts
    11,621
    Thanks
    426

    Re: how to explain?

    Quote Originally Posted by arangu1508 View Post
    how to explain -1 x -1 = +1 (minus x minus equals plus). Is there any practical example (explanation) for this? I have read as a mathematical formula, but how it is true? Kindly enlighten me. Any guidance on this issue is welcome.

    With advance thanks.
    Math Forum: Ask Dr. Math FAQ: Negative Times a Negative
    Thanks from arangu1508
    Follow Math Help Forum on Facebook and Google+

  3. #3
    MHF Contributor
    Prove It's Avatar
    Joined
    Aug 2008
    Posts
    11,492
    Thanks
    1391

    Re: how to explain?

    Quote Originally Posted by arangu1508 View Post
    how to explain -1 x -1 = +1 (minus x minus equals plus). Is there any practical example (explanation) for this? I have read as a mathematical formula, but how it is true? Kindly enlighten me. Any guidance on this issue is welcome.

    With advance thanks.
    Do you know the distributive law? \displaystyle \begin{align*} (a + b) \cdot (c + d) = a\cdot c + a\cdot d + b\cdot c + b\cdot d \end{align*}.

    Let's look at, say, \displaystyle \begin{align*} 9 \cdot 9 = 81 \end{align*}

    We could also write this as

    \displaystyle \begin{align*} 9 \cdot 9 &=  (10 - 1) \cdot (10 - 1) \\ &= 10\cdot 10 + 10\cdot (-1) + (-1) \cdot 10 + (-1)\cdot (-1) \\ &= 100 - 10 - 10 + (-1) \cdot (-1) \\ &= 80 + (-1)\cdot (-1)  \end{align*}

    But we know that \displaystyle \begin{align*} 9\cdot 9 = 81 \end{align*}, so

    \displaystyle \begin{align*} 80 + (-1)\cdot (-1) &= 81 \\ (-1) \cdot (-1) &= 81 - 80 \\ (-1) \cdot (-1) &= 1 \end{align*}


    If we want to visualise this, just picture a 10 x 10 square, and picture evaluating 9 x 9 by removing one row of 10 and one column of 10. The problem is that by doing this, one of the unit squares has been removed twice, and so needs to be put back.
    Thanks from arangu1508
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. can someone explain this
    Posted in the Calculus Forum
    Replies: 2
    Last Post: November 29th 2009, 08:17 AM
  2. Can somebody help explain dx to me?
    Posted in the Calculus Forum
    Replies: 6
    Last Post: November 3rd 2009, 07:20 AM
  3. Explain me why ...
    Posted in the Algebra Forum
    Replies: 3
    Last Post: October 19th 2009, 10:13 AM
  4. Can anyone explain this?
    Posted in the Advanced Statistics Forum
    Replies: 8
    Last Post: March 11th 2009, 08:27 PM
  5. Can someone explain how I do this one?
    Posted in the Trigonometry Forum
    Replies: 1
    Last Post: July 15th 2008, 07:22 PM

Search Tags


/mathhelpforum @mathhelpforum