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November 08, 2025, 05:21:40 am

Author Topic: Inequality  (Read 1037 times)  Share 

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hyperblade01

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Inequality
« on: November 04, 2009, 10:38:56 pm »
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Solve:

2008: Accounting
2009: Chemistry, Biology, Methods CAS, Specialist, English Language
ENTER: 99.10

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THem

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Re: Inequality
« Reply #1 on: November 04, 2009, 10:39:32 pm »
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5/x-3 > -1
5 > -x+3
5-3 > -x
2 > -x
-2< x
?

kamil9876

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Re: Inequality
« Reply #2 on: November 04, 2009, 10:41:42 pm »
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watch out. Multiplying both sides by x-3 can involve multiplying both sides by a positive or negative number, which will change the inequality sign in the latter but not former case.
Voltaire: "There is an astonishing imagination even in the science of mathematics ... We repeat, there is far more imagination in the head of Archimedes than in that of Homer."

krzysiek

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Re: Inequality
« Reply #3 on: November 04, 2009, 10:48:01 pm »
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Kamil, can you expand a tad more on that? How would you identify x-3 as a positive/negative in this case (so you could decide whether to change the inequality brackets)

TrueTears

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Re: Inequality
« Reply #4 on: November 04, 2009, 11:39:35 pm »
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My method:





Case 1:

and

and

Thus

Case 2:

and

and

Thus

Overall: or



Or simply just sketch the graph and work it out.

When in doubt, always sketch a quick graph.
« Last Edit: November 04, 2009, 11:41:24 pm by TrueTears »
PhD @ MIT (Economics).

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THem

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Re: Inequality
« Reply #5 on: November 04, 2009, 11:52:41 pm »
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Graphing works well ;)

hyperblade01

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Re: Inequality
« Reply #6 on: November 04, 2009, 11:53:43 pm »
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Yea thats what led me to this question....because I did what THem did originally but then noticed something wasn't right...
2008: Accounting
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ENTER: 99.10

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TrueTears

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Re: Inequality
« Reply #7 on: November 04, 2009, 11:54:27 pm »
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Yea thats what led me to this question....because I did what THem did originally but then noticed something wasn't right...
Watch out with inequalities, they don't undergo the same operations as your equality signs... xD
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kamil9876

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Re: Inequality
« Reply #8 on: November 05, 2009, 12:26:30 am »
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Kamil, can you expand a tad more on that? How would you identify x-3 as a positive/negative in this case (so you could decide whether to change the inequality brackets)

You can split it into cases, like TT did, assume x-3>0 and get a solution for this domain. Then assume x-3<0 and likewise find the solutions for this domain. Then finally take the union of the two.

Actually looking at his method I think this way is more closer to what Them initially did:


Now assuming we get this:




However we have assumed that x>3, and so we cannot say that all x>-2. But what we have shown is that we require x>-2, which already is certain for all x>3. Hence all x>3 solve the inequality(alternatively this can more cleverly be spotted by noticing that if x>3 then our fraction is positive and hence greater than -1).

Now solving for the other case:




Hence what we have shown is that if x<3, then we require x<-2. Therefore we conclude that all x<-2 solve the inequality.

Taking both cases into account the complete solution is:
Voltaire: "There is an astonishing imagination even in the science of mathematics ... We repeat, there is far more imagination in the head of Archimedes than in that of Homer."