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bmwhype2
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inequalities 03 Mar 2008, 08:18
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given that y>0 and y(y+1) > y(y-4), solve for the possible values of y.

y(y+1) > y(y-4) if i were to cancel out y from both sides, it would result in a different answer. what rule am i volating?

y(y+1) - y(y-4)> 0

ans: y>0



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Capthan
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inequalities 03 Mar 2008, 08:56
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in this example, you can do it, otherwise it is not adviceable.
Because sometimes cancelling out term from each side could result in less factors espesially when your are working with Data sufficiency problem.

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bmwhype2
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inequalities 03 Mar 2008, 12:15
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Yurik79
bmwhype2
given that y>0 and y(y+1) > y(y-4), solve for the possible values of y.

y(y+1) > y(y-4) if i were to cancel out y from both sides, it would result in a different answer. what rule am i volating?

You can cancel out y in this case cause we know that y>0 and the sign of inequality stay the same

y(y+1) - y(y-4)> 0

ans: y>0
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i still dont understand.

if i cancel out y, i have
y+1 > y-4

in order to solve for y, i need to move the y to one side but doing so eliminates the y.
y+1 > y-4
+1 > -4
5 > 0

does this mean that since we know 5>0 is true, the only thing we can infer from y is the given portion in the stem? y>0
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GMATBLACKBELT
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inequalities 03 Mar 2008, 12:32
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bmwhype2
given that y>0 and y(y+1) > y(y-4), solve for the possible values of y.

y(y+1) > y(y-4) if i were to cancel out y from both sides, it would result in a different answer. what rule am i volating?

y(y+1) - y(y-4)> 0

ans: y>0
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Distribute the y's: y^2+y>y^2-4y --> y>-4y --> 0>-5y --> 0<y

Or y>0
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inequalities 26 Mar 2008, 08:50
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Good question.

I think it means that the inequality is true for all values of Y. This plus the given, Y>0, leaves us with the same answer as solving by the less risky method. Y>0.
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inequalities 26 Mar 2008, 13:00
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bmw, I get what you're asking.

the answer is that no, you cannot just divide both sides by a variable in an inequality because of sign changes. EVEN THOUGH we're given that y > 0, mathetically you shouldn't apply that to the equation.
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inequalities 26 Mar 2008, 13:38
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zhenmaster
bmw, I get what you're asking.

the answer is that no, you cannot just divide both sides by a variable in an inequality because of sign changes. EVEN THOUGH we're given that y > 0, mathetically you shouldn't apply that to the equation.
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https://www.urch.com/forums/gmat-math/8 ... ities.html



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