Conventions of Square RootsDate: 11/24/98 at 17:00:35 From: Andrew Subject: Square Roots On a review problem in my Sequential 3A math class, I had the square root of 81 equals positive or negative 9. My math teacher said it was wrong, that if there is no negative before the whole square root problem, only the positive answer is wanted. Who is right? -Andrew Date: 11/24/98 at 17:56:58 From: Doctor Pat Subject: Re: Square Roots Andrew, The convention in mathematics is that x^2 = 81 has two solutions, x = 9 or x = -9, but when we take the square root of a value, we want the principal square root. For real numbers, that is the positive value. It is important that you know that there are two values that may be squared to give every real number (even the negatives), but it is also important that you understand that we may want to make a distinction between all the particular roots of an equation, and the request to give a particular solution. Your teacher clearly wants you to give the positive square root of the value when the square root symbol is used. If situations come up in which there is ambiguity, we need to develop a mutual consent between us as to how we would use certain symbols; otherwise no one would know what I wanted when I wrote 3x4 because you would be unsure what I intended the symbol x to represent. You accept those conventions every day without thinking. You assume problems are in base ten form unless otherwise stated, for example, and that ratios are written with the numerator on top. But once it was common to write 3.4 for 3/4. Hope this helps. - Doctor Pat, 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/