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Inequalities

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Inequalities

by JGaynor » Sun Apr 11, 2010 9:29 am
Hey everyone,

I am having trouble understanding when you want to add or subtract 2 inequality expressions in data sufficiency problems...it seems that sometimes the problem requires adding and sometimes it requires subtracting, or sometimes you just simplify...is there a general rule to know what the best thing to do with inequality expressions are?

for example: If r + s >2t, is r>t?
1) t>s
2) r>s

What gives it away that the expressions should be added to stated problem???

Thank you!

Jason
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by magizhan » Sun Apr 11, 2010 10:16 am
General Rule 1: The inequality sign reverses direction when the eqn is multiplied or divided by a negative number.

General Rule 2: Two inequality eqns can be added if they are of the same signs.

I just apply these two to solve any inequality problem. Applying them to the above qn,

1) t >s implies -s > -t (multiplying both sides by -1). Adding this to the original eqn gives r>t, so SUFFICIENT

2) r > s implies -s > -r. Adding this to the original eqn implies r > 2t - r; 2r > 2t and therefore r > t. so SUFFICIENT.

Answer is D.
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