You are here:

Basic Math/multiplication

Advertisement

Expert: Josh - 4/17/2006


Question
My question is relatively simple. Why is -1 * -1 =+1 ie why minus * minus makes plus?

Answer
Hi Palash,

We generally cannot think of negative numbers as a physical entity. There is no intuitive explanation, as far as I know, why a negative number multiplied by another negative number "should" produce a positive number.

The only explanation I can offer is that by symmetry, we would expect

+a * +b = +ab,
+a * -b = -ab,
-a * +b = -ab, and
-a * -b = +ab.

A rule is a rule. And it is a nice rule.  

Basic Math

All Answers

Answers by Expert:

Ask Experts

Volunteer

Josh

Expertise

When I work through problems, I like to emphasize concepts which I believe are worth noting. I will try to answer questions in the following areas, but not at the advanced level. Algebra. Sequences & Series. Trigonometry. Functions & Graphs. Coordinate Geometry. Quadratic Polynomials. Exponential & Logarithms. Basic Calculus. Probability, Permutation and Combination. Mathematical Induction. Complex numbers. Physics problems.

Experience

I have worked as a teaching assistant in college. My hope is that more people will share knowledge without boundary, give help without seeking recognition or monetary rewards.

Education/Credentials
Bachelor degree in Engineering Science

©2012 About.com, a part of The New York Times Company. All rights reserved.