1
$\begingroup$

I was trying to prove that $-(x + y) = -x - y$ and as you can see in the image below, I took the liberty of using the $-$ symbol as a number and applying the associative law with it. Is it kosher in all rigorousness given the axioms professional mathematicians use? enter image description here

  • 2
    There's a problem in there. You're "pulling out" the $-$ under the "associative law". I guess you know that, given $x$, $(-1) \cdot x =-x$ is its additive inverse. So what you want to do is to use the distributive law as $-(x+y)=(-1)\cdot(x+y)=(-1)\cdot x+(-1)\cdot y$2012-07-10

2 Answers 2

1

$(x+y)+(-x-y)=(y+x)+(-x-y)=y+(x-x)+(-y)=y+0+(-y)=0$ So $(x+y)$ is the additive inverse of $(-x-y)$. Hence $-(x+y)=-x-y$

2

Well the LHS of your equation is just saying "the additive inverse of $x+y$".

So all you have to show is that the additive inverse of $x+y$ really is the RHS of the equation, i.e. $-x-y$, then by uniqueness of inverses in a group the two must be equal.