Product of Additive Inverses

By Susam Pal on 02 May 2020

A negative number multiplied by another negative number results in a positive number. Most of us learnt this rule during our primary or secondary school years. 'Negative times negative equals positive' was a phrase that was drummed into us during mathematics lessons. In this article, we will prove it, not just for numbers but for any algebraic structure that behaves a bit like numbers. We will find out what that means in a moment. First, let us see an illustration of this concept.

Contents

• Illustration
• Ring Axioms
• Closure Properties
• Inverse of Inverse
• Multiplication by Zero
• Multiplication by Additive Inverse
• Product of Additive Inverses
• Conclusion

Illustration

Let us first understand why the product of two negative numbers must be positive for arithmetic to make sense. Consider \[ 7 \times 8 = 56. \] The above equation can also be written as \[ (10 - 3) \times (10 - 2) = 56. \] Using the distributive property of multiplication over subtraction, we get \[ (10 - 3) \times 10 + (10 - 3) \times (-2) = 56. \] Using the distributive property again, we have \[ 10 \times 10 + (-3) \times 10 + 10 \times (-2) + (-3) \times (-2) = 56. \] Now, we will take it for granted that a positive times a negative is negative. We will prove all of this rigorously later, but for now, we are just working through an illustration, so we will accept that rule and see where it leads. The equation becomes: \[ 100 + (-30) + (-20) + (-3) \times (-2) = 56. \] Adding the first three terms gives \[ 50 + (-3) \times (-2) = 56. \] Subtracting \( 50 \) from both sides, we get \[ (-3) \times (-2) = 6. \] What we have seen here is that if we accept \( 7 \times 8 = 56, \) and that positive times negative gives a negative result, then we must also accept that \( (-3) \times (-2) = 6. \)

Ring Axioms

From this section onwards, we begin to take a rigorous approach to everything we discuss. We want to show that the rule 'negative times negative equals positive' holds for any set of elements that share certain properties with numbers. As it turns out, these elements do not need to possess all the properties of complex numbers, real numbers, or even rational numbers. In fact, if they satisfy a small and specific set of properties that integers have, then the rule still holds. These properties are known as the ring axioms.

A ring is an algebraic structure consisting of a set \( R \) with two binary operations \( + \) and \( \cdot, \) called addition and multiplication respectively, that satisfy the following axioms:

1. Associativity of addition: For all \( a, b, c \in R, \) we have \( a + (b + c) = (a + b) + c. \)

2. Commutativity of addition: For all \( a, b \in R, \) we have \( a + b = b + a. \)

3. Additive identity: There exists an element \( 0 \in R \) such that for all \( a \in R, \) we have \( a + 0 = a. \)

4. Additive inverse: For each \( a \in R, \) there exists an element \( -a \in R \) such that \( a + (-a) = 0. \)

5. Associativity of multiplication: For all \( a, b, c \in R, \) we have \( a \cdot (b \cdot c) = (a \cdot b) \cdot c. \)

6. Left distributivity of multiplication over addition: For all \( a, b, c \in R, \) we have \( a \cdot (b + c) = (a \cdot b) + (a \cdot c). \)

7. Right distributivity of multiplication over addition: For all \( a, b, c \in R, \) we have \( (b + c) \cdot a = (b \cdot a) + (c \cdot a). \)

Note that we do not assume that the ring contains multiplicative identity, nor do we assume that multiplication is commutative. Many familiar types of numbers form rings. For example, the set of integers forms a ring with the usual addition and multiplication operations. The sets of rational numbers, real numbers, and complex numbers satisfy the ring axioms too.

Rings need not consist of numbers. A ring may contain elements of any type. As long as they satisfy the seven axioms above, they form a ring. For example, the set of all polynomials in the indeterminate \( t \) with coefficients in another ring \( R \) forms a ring under the usual addition and multiplication of polynomials. Such a ring is called a polynomial ring and it is denoted \( R[t]. \)

Closure Properties

Some texts include the following additional axioms for the closure properties of a ring:

1. Closure under addition: For all \( a, b \in R , \) we have \( a + b \in R. \)

2. Closure under multiplication: For all \( a, b \in R, \) we have \( a \cdot b \in R. \)

However, stating these axioms explicitly is usually considered redundant because a binary operation is closed by definition. A binary operation \( \circ \) on a set \( M \) is defined to be a function \[ \circ : M \times M \to M; \quad (a, b) \mapsto a \circ b. \] This definition automatically implies the closure property. The addition and multiplication operations on a ring \( R \) may be defined as \begin{align*} + &: R \times R \to R; \quad (a, b) \mapsto a + b, \\ \cdot &: R \times R \to R; \quad (a, b) \mapsto a \cdot b. \end{align*} These definitions imply that a ring is closed under addition and multiplication. In practice, while deciding if some set \( R \) forms a ring, we should always verify that the addition and multiplication operations indeed have \( R \) as the codomain to confirm that the closure property holds.

Inverse of Inverse

Theorem 1. Let \( R \) be a ring with \( + \) and \( \cdot \) operations. Then for all \( a \in R, \) we have \[ -(-a) = a. \]

Proof. Using the additive inverse axiom, we get \[ a + (-a) = 0. \] Let \( b = -a \) and rewrite the equation as \[ a + b = 0. \] Adding \( -b \) to both sides, we get \[ (a + b) + (-b) = 0 + (-b). \] Using associativity of addition in a ring, we get \[ a + (b + (-b)) = 0 + (-b). \] By the additive inverse axiom, we obtain \[ a + 0 = 0 + (-b). \] Using the additive identity axiom along with the commutativity of addition in a ring, we get \[ a = -b. \] Since \( b = -a, \) the above equation can be written as \[ a = -(-a). \] This completes the proof.

Multiplication by Zero

Theorem 2. Let \( R \) be a ring with \( + \) and \( \cdot \) operations. Then for all \( a \in R, \) we have \[ a \cdot 0 = 0 \cdot a = 0. \]

Proof. Using the additive identity axiom, we get \[ 0 + 0 = 0. \] Multiplying both sides on the left by \( a, \) we get \[ a \cdot (0 + 0) = a \cdot 0. \] Using the left distributivity axiom, we get \[ a \cdot 0 + a \cdot 0 = a \cdot 0. \] Let \( b = a \cdot 0. \) Then \[ b + b = b. \] Since a ring is closed under multiplication, \( b \in R. \) By the additive inverse axiom, there exists \( -b \in R \) such that \( b + (-b) = 0. \) Adding \( -b \) to both sides of the above equation, we get \[ (b + b) + (-b) = b + (-b). \] By associativity of addition in a ring, we get \[ b + (b + (-b)) = b + (-b). \] Since \( b + (-b) = 0, \) we get \[ b + 0 = 0. \] By the additive identity axiom, we get \[ b = 0. \] Since \( b = a \cdot 0, \) the above equation may be written as \[ a \cdot 0 = 0. \] A similar argument shows that \[ 0 \cdot a = 0. \] This completes the proof.

Multiplication by Additive Inverse

Theorem 3. Let \( R \) be a ring with \( + \) and \( \cdot \) operations. Then for all \( a, b \in R, \) we have \[ a \cdot (-b) = (-a) \cdot b = -(ab). \]

Proof. Using the left distributivity and additive inverse properties of a ring along with Theorem 2, we get \[ a \cdot b + a \cdot (-b) = a \cdot (b + (-b)) = a \cdot 0 = 0. \] Adding \( -(a \cdot b) \) to both sides, we get \[ -(a \cdot b) + (a \cdot b + a \cdot (-b)) = -(a \cdot b). \] By associativity of addition, we obtain \[ (-(a \cdot b) + a \cdot b) + a \cdot (-b) = -(a \cdot b). \] Using commutativity of addition in a ring along with the additive inverse axiom, we get \[ 0 + a \cdot (-b) = -(a \cdot b). \] Finally, using commutativity of addition in a ring along with the additive identity axiom, we get \[ a \cdot (-b) = -(a \cdot b). \] Similarly, we can show that \[ (-a) \cdot b = -(a \cdot b). \] This completes the proof.

Product of Additive Inverses

Theorem 4. Let \( R \) be a ring with \( + \) and \( \cdot \) operations. Then for all \( a, b \in R, \) we have \[ (-a) \cdot (-b) = ab. \]

Proof. From Theorem 3, we know that \[ a \cdot (-b) = -(a \cdot b). \] Substituting \( a \) with \( -a, \) we get \[ (-a) \cdot (-b) = -((-a) \cdot b). \] Again by Theorem 3, we have \( (-a) \cdot b = -(a \cdot b). \) This gives \[ (-a) \cdot (-b) = -(-(a \cdot b)). \] Now using Theorem 1, the right-hand side becomes \( a \cdot b, \) so we get \[ (-a) \cdot (-b) = a \cdot b. \] This completes the proof.

Conclusion

Theorems 1 to 4 establish certain algebraic properties that hold in any ring. Although these results were proven abstractly for rings, they reflect properties we are already familiar with from our experience with numbers. For example, in the ring of integers, we observe \( -(-2) = 2 \) which is a specific case of Theorem 1.

Similarly, Theorem 2 confirms the well-known fact that multiplying any integer by \( 0 \) yields \( 0 . \) For example, \( 2 \cdot 0 = 0. \)

Then Theorem 3 implies the rule that multiplying a positive number by a negative number yields a negative result. For example, \( 2 \cdot (-3) = -(2 \cdot 3) = -6. \)

Finally, Theorem 4 implies that the product of two negative numbers is positive. For example, \( (-2) \cdot (-3) = 2 \cdot 3 = 6. \)

These familiar results are not limited to the integers. The results hold in any ring, including polynomial rings, rings of integers modulo a fixed positive integer, and many other algebraic systems. These results offer a glimpse of how the ring axioms can formalise familiar arithmetic rules within a more general algebraic framework.