Important Topic!

Back when I was in school, this concept was really hard for me to grasp my head around, and I must say, this was a compelling way to understand the function of multiplying with negative numbers!

Thanks!

By virtue of everyday usage, the fact that (-1) x (-1) = 1 has been engraved onto our heads. But, only recently did I actually sit down to explore why, in general negative times negative yields a positive number !

Intuition.

Let’s play a game called “continue the pattern”. You would be surprised, how intuitive the results are:

2 x 3 = 6

2 x 2 = 4

2 x 1 = 2

2 x 0 = 0

2 x (-1) = ??  (Answer : -2 )

2 x (-2 ) = ?? (Answer : -4 )

2 x ( -3) = ?? (Answer : -6 )

The number on the right-hand side keeps decreasing by 2 !

Therefore positive x negative = negative. 

2 x -3 = -6

1 x -3 = -3

0 x -3 = 0

-1 x -3 = ?? (Answer : 3)

-2 x -3 = ?? (Answer : 6)

The number on the right-hand side keeps increasing by 3.

Therefore negative x negative = positive.

Pretty Awesome, right? But, let’s up the ante and compliment our intuition.

The Number Line Approach.

Imagine a number line on which you walk. Multiplying x*y is taking x steps, each of size y.

Negative steps require you to face the negative end of the line before you start walking and negative step sizes are backward (i.e., heel first) steps.

So, -x*-y means to stand on zero, face in the negative direction, and then take x backward steps, each of size y.

Ergo, -1 x -1 means to stand on 0, face in the negative direction, and then take 1 backward step. This lands us smack right on +1 !

The Complex Numbers Approach.

The “i” in a complex number is an Instruction! An instruction to turn 90 degrees in the counterclockwise direction. Then i * i would be an instruction to turn 180 degrees. ( i x i = -1 ). where i = √-1

Similarly ( -1 ) x i x i = (- 1 ) x ( -1 )= 1. A complete revolution renders you back to +1.

We can snug in conveniently with the knowledge of complex numbers. But, complex numbers were established only in the 16th century and the fact that negative time negative yields a positive number was well established before that.

Concluding remarks.

Hope you enjoyed the post and Pardon me if you found this to be rudimentary for your taste. This post was inspired by Joseph H. Silverman’s Book - A friendly Introduction to Number Theory. If you are passionate about numbers or math, in general it is a must read.

There are several other arithmetic methods that prove the same, if you are interested feel free to explore.

Have a Good Day!

PC: mathisfun

Glad to see your back. Love your work. As always, keep up the good job! And btw, nice covergirl photo ;)

Rough draft of a sculpture illustrating the electric field lines between two equal and opposite charges.

(Hi! I’m still alive, sorry for ghostin ya tumblr. I stopped posting because my reclusive tendencies have gotten really out of control over the past year or so, but I’m trying to take little baby steps to quittin my bullshit so bear with me! Still makin math art like crazy, so I thought I’d start with sendin u this lil ditty I whipped up a couple weeks ago. Plenty more where this came from, I promise!)

Greenpeace #clickclean campaign GIFs ‘Wind Turbine Likes’

This beef just got REAL!

NDT just murdered B.o.B.

Three of my favorite tumblrs in one post, keep it up boys. :)

Dog Chase - Switcher

I saw Regolo Bizzi’s wonderful drawings on twitter, and I had to try and make them in processing. I recognized it as a version of the dog chase problem (where dogs at each corner of a square are chasing the dog to their left), but now they switch from the left dog to the right dog every 100 frames. Also made a hexagon version (6 dogs), because why not?

Square code: http://openprocessing.org/sketch/292065

Polygon code: http://www.openprocessing.org/sketch/292076

Wow, it’s somehow really relaxing in this!

Koalas to the Max

Mike Lawler and Dave Radcliffe reminded me of this great goofy site: Koalas to the Max. 4x the fun of your usual dots site.

Master of geogebra helped me out with this! Thanks!

Table Saw

@mrvmt requested a table saw sketch, and this is the first draft. In GeoGebra here.

Down the rabbithole. :)

/|\ / \ hello it is I, twocubes

(via https://www.youtube.com/watch?v=0C75vRVL5lE)

One of the best videos on youtube!

Oh, so your from Denmark then, or maybe its “Skåne” ? :)