Left AnglesDate: 06/01/99 at 22:13:07 From: greg Subject: Geometry What is a left angle? I know it is not a right angle and is not 90 degrees. Our math teacher will give us a prize if we can get this question right. Please tell me! Date: 06/02/99 at 08:26:44 From: Doctor Peterson Subject: Re: Geometry Hi, Greg. There's no such thing as a left angle, unless someone has recently invented it just for fun. But this is a fascinating question for those of us who like both words and math. The word "right" has so many meanings that it can be confusing to figure out which one to pay attention to; a "right angle" has nothing to do with right- or left- handedness. Think about all the different things "right" can mean: As an adjective or adverb: correct, true, proper ("the right thing to do") opposite of left ("my right hand") perpendicular ("a right angle") straight ("right toward the goal") As a verb: correct ("righting wrongs") make upright ("right the boat") As a noun: privilege ("I have a right to ...") It's related to the Latin adjective "rectus," which is the past participle of the verb "rego" meaning "to direct, lead, or rule"; from these words we also get such words as "direct," "correct," "erect," "rectangle," "rectify," and "rectitude" (as well as "regulate" and "regal"). "Rectus" has pretty much the same range of meaning as "right" in English, as you can tell from this list: straight, upright, perpendicular, correct, proper. The Greek equivalent is "orthos," which again means either "straight," "perpendicular," or "correct." Your orthodontist, of course, "straightens" your teeth, and "orthogonal" means a right angle. The basic meaning is "straight"; if you take this as "standing straight," it means "upright" or "vertical," which is where we get the meaning of perpendicular: a right angle is one that stands up straight. This was the word used by Euclid in geometry. Take "upright" figuratively, and you get the moral meaning of "upright"; and since a straight line isn't crooked and goes where it is supposed to, it is also "correct." So I would say that we call a right angle "right" because it is "upright" or "erect." The real mystery is, why is the right hand called right? That's where the bias against left-handers comes in. Greek and Latin both have words for the right side (dexios, dexter) that are unrelated to their words for "straight." So it must be in English or its precursors that right-handedness became "right" - the "correct" hand to use if you want "dexterity." Here's a diagram to show how the various meanings are related: +--> perpendicular / straight ---> upright ---> proper \ +---------------> correct ---------> right (hand) \_______________________________________/ orthos, rectus \________________________________________________________/ right Since a "right" angle has nothing to do with left and right, there's no reason for a "left" angle to exist. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/ |
Search the Dr. Math Library: |
[Privacy Policy] [Terms of Use]
Ask Dr. Math^{TM}
© 1994- The Math Forum at NCTM. All rights reserved.
http://mathforum.org/dr.math/