Equations That Make No Sense
Date: 01/15/2003 at 00:16:37 From: Nicholas Subject: Rational Equations x-8/5 = x+2/6 I would assume you find the lowest common denominator of 5 and 6. Then multiply it by x. Not sure if it's correct though.
Date: 01/16/2003 at 11:30:46 From: Doctor Ian Subject: Re: Rational Equations Hi Nicholas, You could certainly proceed by finding a common denominator, but it would be kind of like driving to New York on your way from Chicago to Los Angeles. Let's write the equation in a slightly fuzzier way: x - this = x + that Think about the number line for a moment. Suppose we choose a number on it, somewhere: <-------------|-------------> x If we subtract something from it, we end up to the left of where we started: <---|----------|-------------> x-this x And if we add something to it, we end up to the right of where we started: <---|----------|---------|----> x-this x x+that Is there _anywhere_ on the number line that we could start so that these two new points are in the same place? This is what we're asking when we write the equation x - this = x + that There is no such place. So there is _no_ value of x that will make this equation true. How can we show that 'by algebra'? We can start by noting that we can subtract the same thing from both sides of an equation. Why not subtract x from both sides? Then we get x - 8/5 - x = x + 2/6 - x -8/5 = 2/6 Now, this clearly isn't true. But we derived it from the original equation using valid rules. So what does that mean? It means that the original equation can't be true, either. As I said, we could also proceed by finding a common denominator for the fractions: x - 8/5 = x + 2/6 x - 48/30 = x + 10/30 x - 48/30 + 48/30 = x + 10/30 + 48/30 x = x + 58/30 And again, we have something that obviously can't be true. There is no number such that you can add a non-zero number to it, and end up with the same thing. So what should you learn from this? One important lesson, I think, is that it's possible to write equations that make no sense, just as it's possible to write sentences that make no sense: Context, Language, and False Equations http://mathforum.org/library/drmath/view/52359.html A second important lesson is that before you jump in and start executing the first technique that occurs to you, it's a good idea to step back and think about whether there might be an easier way to get the answer you're looking for: Factoring vs. an Equation http://mathforum.org/library/drmath/view/61333.html I hope this helps. Write back if you'd like to talk more about this, or anything else. - Doctor Ian, The Math Forum http://mathforum.org/dr.math/
Search the Dr. Math Library:
Ask Dr. MathTM
© 1994-2013 The Math Forum