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are x , y > 0 ? 1) 2x-2y =1 2) x/y > 1 should we

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are x , y > 0 ? 1) 2x-2y =1 2) x/y > 1 should we [#permalink]

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New post 19 Jul 2008, 22:58
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are x , y > 0 ?

1) 2x-2y =1
2) x/y > 1

should we always solve DS q's with taking sample numbers ? this q's is prety tricky if I try to solve the eqn , ineq.

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Re: should we always solve DS q's with taking sample numbers ? [#permalink]

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New post 19 Jul 2008, 23:08
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1) Tells us 2x-2y =1. This tells us definitely that x > y however both x and y can greater, eqaul to or less than 0. So A and D out.

2) x/y > 1
This tells us x > y if y is +ve (which means x is +ve as well) Or x < y is y is -ve (which means x is -ve as well). Again two possible values so not possible.

Combining we have x > y from 1 and 2 says x > y only if x and y are positive. So Answer C.

However having solved that, I will concur that it is pretty trick problem and for most of the people picking number is better strategy.

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Re: should we always solve DS q's with taking sample numbers ? [#permalink]

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New post 19 Jul 2008, 23:36
I would do the problem exactly as abhijit did above. To this question:

"should we always solve DS q's with taking sample numbers ?"

I would say no, not 'always'. I'd only try to solve DS questions with sample numbers if you cannot solve the problem conceptually or algebraically. It is often the case that if you don't pick a sufficient variety of sample numbers, you will reach an incorrect conclusion, and therefore a wrong answer.
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Re: should we always solve DS q's with taking sample numbers ? [#permalink]

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New post 19 Jul 2008, 23:47
some people get it and some people dont but i feel that if you have two variablee inequality DS problem, plotting the conditions on xy plane is the best method.. only thing is, dont try this in exam without practise

question : x,y > 0 ... target area is Q1

statement 1 : line passes through Q1, Q2 and Q3 ... not suff
statement 2 : green area ... in Q1 and Q3 .. not suff

combine, line segament in blue, alwasy in Q1 ...suff ..answer
Attachment:
DSQ3.JPG
DSQ3.JPG [ 9.55 KiB | Viewed 1000 times ]

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Re: should we always solve DS q's with taking sample numbers ? [#permalink]

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New post 20 Jul 2008, 00:10
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Let take one more example :
On number line , the distance between x and y is greater than distance between x and z. does z lie between x and y ?

1) xyz <0
2) xy<0

Now this resolves to is |X-Y| > |X-Z| ?

lets try solve algebracally and by sample data ... and see how much time each takes ...

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Re: should we always solve DS q's with taking sample numbers ? [#permalink]

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New post 20 Jul 2008, 00:33
abhaypratapsingh wrote:
Let take one more example :
On number line , the distance between x and y is greater than distance between x and z. does z lie between x and y ?

1) xyz <0
2) xy<0

Now this resolves to is |X-Y| > |X-Z| ?

lets try solve algebracally and by sample data ... and see how much time each takes ...


is it E

statement 1 ... clearly not suff ... x,y,z all can be -ve and you never know z was on which side of x
statement 2 ... not suff ... doesn tell anything about Z

combine : ...z is Positive
and one of x or y is negative ....

if y is negative .... both x and z are positive and z can be on either side of x ... not suff

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Re: should we always solve DS q's with taking sample numbers ? [#permalink]

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New post 30 Jul 2008, 16:27
durgesh79 wrote:
some people get it and some people dont but i feel that if you have two variablee inequality DS problem, plotting the conditions on xy plane is the best method.. only thing is, dont try this in exam without practise

question : x,y > 0 ... target area is Q1

statement 1 : line passes through Q1, Q2 and Q3 ... not suff
statement 2 : green area ... in Q1 and Q3 .. not suff

combine, line segament in blue, alwasy in Q1 ...suff ..answer
Attachment:
DSQ3.JPG


Isn't the green area colored incorrectly? If x/y > 1, that means that x>y. The graph should than be colored such that everything to the right of the line y=x is green. Am I missing something?

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Re: should we always solve DS q's with taking sample numbers ? [#permalink]

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New post 30 Jul 2008, 18:46
mrblack wrote:
Isn't the green area colored incorrectly? If x/y > 1, that means that x>y. The graph should than be colored such that everything to the right of the line y=x is green. Am I missing something?


If x/y > 1, that means that:

x > y if y > 0
x < y if y < 0

Remember, we need to reverse the inequality if we multiply both sides by a negative number, and y can certainly be negative.
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Re: should we always solve DS q's with taking sample numbers ? [#permalink]

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New post 30 Jul 2008, 18:52
IanStewart wrote:
mrblack wrote:
Isn't the green area colored incorrectly? If x/y > 1, that means that x>y. The graph should than be colored such that everything to the right of the line y=x is green. Am I missing something?


If x/y > 1, that means that:

x > y if y > 0
x < y if y < 0

Remember, we need to reverse the inequality if we multiply both sides by a negative number, and y can certainly be negative.


Thanks. That's a very good point. I got it now.

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Re: should we always solve DS q's with taking sample numbers ?   [#permalink] 30 Jul 2008, 18:52
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