Here we need to see which axioms or properties are used in each case.
A: ab²×0 = 0
Here we have a given number (ab²) times zero, and the outcome is zero. This comes from the existence of zero, which states that there must exist a value that represents nothingness, the zero, such that the product between any number and this one is zero.
B: (7 + 5) + 1 = 7 + (5 + 1)
This is the associative property of addition, which says that we can perform the addition in any order we want.
C: 6 = 6
This is the reflexive property of equality, which says that every number is equal to itself.
D: 7(x – 3) = 7x - 21
This is the distributive property, which says that:
C*(A + B) = C*A + C*B
We can see that if we apply it to the left side we get:
7(x - 3) = 7x - 7*3 = 7x - 21
E: x + (-x) = 0
This is the existence of the opposite number. This property says that for any real number N there exists a real number A such that:
N + A = 0
Particularly, in the real set, we can see that A = -N
G: (a + b) + 0 = 0 + (a + b)
Here we can see that the order changes in the addition, this is the commutative property of the addition, which says that:
A + B = B + A
So the elements in the sum can commute.
H: if x = -1, and -1 = z, then x = z
This is the transitive property, we can apply it in the next way:
x = -1 = z
then x = z
So if both variables are equal to the same number, then the variables are equal.
I: 4x×1 = 4x
This is the existence of the identity with respect to multiplication. It says that there exist a number A such that for any number X, the product between A and X is equal to X:
X*A =X
Particularly, A = 1.
X*1 = X.
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