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# I'm very confused with cross multiplying inequalities. Is

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Manager
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I'm very confused with cross multiplying inequalities. Is [#permalink]  22 Jun 2008, 23:11
I'm very confused with cross multiplying inequalities. Is there a general rule as to when it can only be allowed? Would really appreciate tips. Thanks
Manager
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Re: Inequality cross multi question [#permalink]  22 Jun 2008, 23:16
When b & y are positive: the ineq a/b > x/y can be written as ay>bx as the sign remains the same

But when any one or both of b & y are negative; ay<bx

Thumbrule: Any inequality when multiplied with a negative number changes the direction of inequality and when multiplied with a +ve number an inequality maintains the direction of the ineq.

HtH
Manager
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Re: Inequality cross multi question [#permalink]  22 Jun 2008, 23:32
So if the denominator is a variable, then, better not to cross multiply?
Here's a DS example from OG C
Question is: If x is not equal to -y, is (x-y)/(x+y) >1?
In this equation, I cannot simplify it to (x-y)>(x+y) right?because denominator (x+y) is unknown?
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Re: Inequality cross multi question [#permalink]  23 Jun 2008, 00:01
Yes as long as you dont know the sign of x+y!
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Re: Inequality cross multi question [#permalink]  23 Jun 2008, 10:07
Nice website to learn some basic stuff...
http://www.purplemath.com/modules/index.htm

search for inequalities ....
Re: Inequality cross multi question   [#permalink] 23 Jun 2008, 10:07
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