Fractions marred in inequality

by Simon 6. September 2010 21:32

Ah, inequality. That terrible, terrible thing. What does one do when confronted with inequality? Well, usually we march, protest, or legislate against it. What do mathematicians do? We work out its domain and range.

So what's stumping me tonight? Well, many things are stumping me tonight, but I feel like I'm getting into my groove. What in particular is stumping me however is this:

So I multiply both sides by the denominator:

Which gives me:

That, my friends, is not a true statement. So what did I do wrong?

Tags:

Fractions | Maths | Inequalities

Comments

9/6/2010 11:49:53 PM #

solakv

It has been a while since I dealt with these things, but let's start here:
There's a boundary between this being true and being false. That boundary is at:

(10x ÷ (2x + 3)) = 5

If we solve for that, then go back and figure out which side satisfies the inequality, that will save us having to deal with the inequality for the whole time.

As you did above, we go along …

10x = 5 (2x + 3)
10x = 10x + 15
0 = 15
false.
So I would have to guess that this thing is never true, but as I said, it's been a while, so I'll keep thinking.

I was also going to worry about when (2x + 3) is zero, because dividing by that will cause trouble, but that only happens at x=-1.5 and should be fine everywhere else.

The other thing to worry about is when (2x + 3) is negative, because multiplying both sides by that will require reversing the inequality. Aha! Give that a try: whenever you multiply by a term that's (even partially) unknown, you have to split your reasoning into two paths, for "if the new factor is positive" and "if it's negative".

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About the author

A 23 year old back in school. I thought I was finished with maths when I left high school. I was wrong. This is my journey of how I have to relearn all that I've forgotten, and the road ahead from introductory algebra, all the way to, well, wherever college math ends, I suppose!

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