Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why negative times negative is positive

  1. Feb 24, 2012 #1
    hey friend can anybody give answer?
    why (-) * (-) = (+)
     
  2. jcsd
  3. Feb 24, 2012 #2

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    Re: Question

    Its impossible to give an explanation that you would accept without knowing what kind of explanation you would accept!

    For example, one can show that a(b+ c)= ab+ ac (the "distributive law"). In particular, if we take a=-x, b=y, c= -y, that says -x(y+ (-y))= -x(y)+ (-x)(-y). But y+(-y)= 0 and -x(0)= 0 so that tells us that -x(y)+ (-x)(-y)= 0. Adding x(y) to both sides, -x(-y)= x(y).

    Is that acceptable?
     
  4. Feb 24, 2012 #3

    dx

    User Avatar
    Homework Helper
    Gold Member

    Re: Question

    Think of -a as a reflection of a through the origin. Reflecting twice gives you back a: -(-a) = a.
     
  5. Feb 25, 2012 #4
  6. Feb 25, 2012 #5
    :rofl:

    nice link
     
  7. Feb 26, 2012 #6
    I have a simple explanation if u confined to Integers only
    first of u need to know why - * + = -
    multiplication is seemed to be derived from addition. Like2*3=2+2+2. and 3+4=3+3+3+3..
    So when u write (-3)*4= (-3)+ (-3)+ (-3)+ (-3)=- (-12)
    hence it is proved that (-)* (+)= (-)

    Now come to (-)* (-)= (+)
    Let me say u want to solve the question something like this. (-1)/1. you must have done divisions in primary classes where you make make a pie ∏ shape. write denominator part inside and numerator part outside etc,..
    when u do such ting with (-1)/1. then u have two choices for quotient 1 or (-1). If u put 1 then using quotient rule. 1*(-1)+0=1 . But it's wrong so -1 is only answer u can think of..

    All this is poor explanation since real numbers and complex number are not taken.
    these all are based on logics and data.2+(3+5)=2*8=2*3+2*5 after many tries it was found that is applicable everywhere so it's made as property of numbers.
    similar about others...
     
  8. Feb 26, 2012 #7
  9. Feb 29, 2012 #8
    mathforum is also not bad..
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Similar Discussions: Why negative times negative is positive
  1. Negatives and positives (Replies: 32)

Loading...