• September 19th 2009, 03:37 PM
skeeterrr
Hi, I'm in first year university and I'm new to proofs, so I was wondering if anyone here can help me out.

1) A + (B + C) = (A + B) + c

and

2) For each m x n matrix A, there is unique m x n matrix D such that

A + D = 0

Any hints and insights would be greatly appreciated!
• September 20th 2009, 03:11 AM
davidlyness
Let's assume you're working over the real numbers.

Let $A = \left( {{a_{ij}}} \right)$, $B = \left( {{b_{ij}}} \right)$ and $C = \left( {{c_{ij}}} \right)$. Define $D = A + (B + C)$ and $E = (A + B) + C$.

By definition of the addition of matrices, we have ${d_{ij}} = {a_{ij}} + ({b_{ij}} + {c_{ij}}) = ({a_{ij}} + {b_{ij}}) + {c_{ij}} = {e_{ij}}$, since real numbers are commutative. So, $D = E$, and equivalently $A + \left( {B + C} \right) = \left( {A + B} \right) + C$.

As for your second problem, letting $A = \left( {{a_{ij}}} \right)$, then set $D = \left( {{d_{ij}}} \right) = \left( { - {a_{ij}}} \right)$ (i.e. each entry of matrix D is the additive inverse of the same entry in A). Then it follows that A + D is equal to the zero matrix, and you're done.
• September 23rd 2009, 06:55 AM
davidlyness
Futhermore, if you want to show that there is a UNIQUE matrix for 2), assume there are two different matrices which satisfy the condition, and work through all logical steps you can take - you'll get that they are equal to each other, and so your result is unique.
• September 23rd 2009, 02:39 PM
skeeterrr
I think I understand now. Thanks for the help!