Ineq...

So in an ineq. question, we cannot do something like:

x-y / x+y > 1

to

x-y > x+y?

what is the rule for something like this? Do we treat it kind of like absolute values?

Ancientmtk wrote:So in an ineq. question, we cannot do something like:

x-y / x+y > 1

to

x-y > x+y?

what is the rule for something like this? Do we treat it kind of like absolute values?

We need to be very careful when we manipulate inequalities, especially when variables are involved. Remember, if you multiply or divide both sides by a negative, you need to reverse the inequality.

So, unless we know the sign of (x + y), we can't simply "move it over", since "moving it over" is really the result of multiplying both sides by (x + y).

You could set up two solutions:

if x + y > 0, then we get:

x - y > x + y
0 > 2y
0 > y

if x + y < 0, however, then we get:

x - y < x + y
0 < 2y
0 < y

Hi Stuart, I did the same mistake as well by only considering the postive values of the fraction.

What would be easiest way to go about these kind of questions? Picking numbers or trying to put algebrc equations , considering both +ve and -ve possibilites of these variables.

ildude02 wrote:Hi Stuart, I did the same mistake as well by only considering the postive values of the fraction.

What would be easiest way to go about these kind of questions? Picking numbers or trying to put algebrc equations , considering both +ve and -ve possibilites of these variables.

That's a really big question and the answer applies to pretty much all of GMAT math.

"Easiest" will depend on your own mathematical expertise. If you see a question and the algebra jumps off the page at you, then using algebra will probably be the quickest approach. On the other hand, if your first reaction after reading a question is "whaaaaaaaa!!", then you need to find an alternative approach for that question.

Since DS is all about understanding mathematical principles, it's really a better test of understanding than is problem solving. Use your performance in DS to judge how well you understand the various rules that the GMAT tests and to help focus your study where you need it the most.

In DS, the best alternative to "just understanding" is almost always picking numbers and seeing what happens. Like any other skill, picking numbers takes time to master (especially since we rarely use it in everyday life - if your boss tells you to write a report, picking numbers and seeing what happens probably won't get the job done).

I advise all my students that, in untimed practice, if they see 3 different ways to try a question they should try all 3. Practicing alternative approaches has 2 main benefits:

(1) you get better and better at those approaches; and

(2) you learn which approaches work best for which questions.