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Re: If y=−m^2, which of the following must be true? [#permalink]
01 Jun 2012, 01:20

10

This post received KUDOS

Expert's post

Joy111 wrote:

If y=−m^2, which of the following must be true?

I. y is negative. II. m is non-negative. III. If m is negative then y is negative.

A. I only B. II only C. III only D. I and II only E. II and III only

Tricky question. +1.

If y=−m^2, which of the following must be true?

First of all notice m^2 is always non-negative, so -m^2 is non-positive (zero or negative), which means that y is zero when m=0 and y is negative for ANY other value of m.

I. y is negative --> not necessarily true, if m=0 then y=-m^2=0;

II. m is non-negative. m can take ANY value: positive, negative, zero. We don't have any restrictions on its value;

III. If m is negative then y is negative. m is negative means that m is not zero. As discussed above if m is other than zero (positive or negative) then y is negative: y=-(negative^2)=-positive=negative (y=-(positive^2)=-positive=negative). So, this option is always true.

Re: If y=−m^2, which of the following must be true? [#permalink]
01 Jun 2012, 02:14

Expert's post

sorry bunuel for the question.

I pick the right answer in less than minute (practice helps). As such, the second choice when I read m is NON negative I think to 0 or positive, but you say "ANY value" even negative. I'm the first time that encounter this nuance. Can you give me some thoughts ???

Re: If y=−m^2, which of the following must be true? [#permalink]
01 Jun 2012, 02:24

Expert's post

carcass wrote:

sorry bunuel for the question.

I pick the right answer in less than minute (practice helps). As such, the second choice when I read m is NON negative I think to 0 or positive, but you say "ANY value" even negative. I'm the first time that encounter this nuance. Can you give me some thoughts ???

Thanks

I say that from y=−m^2 we don't have ANY restriction on the value of m, so it can take ANY value: positive, negative or zero. So, option II which says that "m is non-negative" (read zero or positive) is not always true since m can also be negative.

Re: If y=−m^2, which of the following must be true? [#permalink]
01 Jun 2012, 03:03

Bunuel wrote:

Joy111 wrote:

If y=−m^2, which of the following must be true?

I. y is negative. II. m is non-negative. III. If m is negative then y is negative.

A. I only B. II only C. III only D. I and II only E. II and III only

Tricky question. +1.

If y=−m^2, which of the following must be true?

First of all notice m^2 is always non-negative, so -m^2 is non-positive (zero or negative), which means that y is zero when m=0 and y is negative for ANY other value of m.

I. y is negative --> not necessarily true, if m=0 then y=-m^2=0;

II. m is non-negative. m can take ANY value: positive, negative, zero. We don't have any restrictions on its value;

III. If m is negative then y is negative. m is negative means that m is not zero. As discussed above if m is other than zero (positive or negative) then y is negative: y=-(negative^2)=-positive=negative (y=-(positive^2)=-positive=negative). So, this option is always true.

Answer: C (III only).

Hope it's clear.

If m is positive then too y is negative so statement iii is contradicted ?

Re: If y=−m^2, which of the following must be true? [#permalink]
01 Jun 2012, 03:42

1

This post received KUDOS

Joy111 wrote:

If m is positive then too y is negative so statement iii is contradicted ?

In response to the doubt (also on PM)

Joy the question is asking which must always be true

I will go thru the conditions 1 and 2 once again to help you understand the condition 3

Condition 1 says "y is -ve" That means it is saying that "y=-m^2" is always -ve But if m=0 then y becomes 0. So condition 1 is not true

We rule out choices A and D

Condition 2 says "m is non-negative" That means it is saying that in "y=-m^2" m is either 0 or +ve But it is not true, m can be -ve also From the question stem, there is no restriction on value of m, and hence m can be any number

We rule out choices B and E

solved Answer is C

Pursuing further Codition 3: "If m is negative then y is negative." This condition limits the values of m It is saying that IF m<0 then y is always -ve The only time when y is not -ve is when m=0 as the condition says that m is less than 0 (that is to say that m is not 0)

Hence when m is -ve (less than 0) y is -ve Always True

Re: If y=−m^2, which of the following must be true? [#permalink]
01 Jun 2012, 13:33

manulath wrote:

Joy111 wrote:

If m is positive then too y is negative so statement iii is contradicted ?

In response to the doubt (also on PM)

Joy the question is asking which must always be true

I will go thru the conditions 1 and 2 once again to help you understand the condition 3

Condition 1 says "y is -ve" That means it is saying that "y=-m^2" is always -ve But if m=0 then y becomes 0. So condition 1 is not true

We rule out choices A and D

Condition 2 says "m is non-negative" That means it is saying that in "y=-m^2" m is either 0 or +ve But it is not true, m can be -ve also From the question stem, there is no restriction on value of m, and hence m can be any number

We rule out choices B and E

solved Answer is C

Pursuing further Codition 3: "If m is negative then y is negative." This condition limits the values of m It is saying that IF m<0 then y is always -ve The only time when y is not -ve is when m=0 as the condition says that m is less than 0 (that is to say that m is not 0)

Hence when m is -ve (less than 0) y is -ve Always True

I think both of you are correct, I was thinking the question to be " Which of the following is ALWAYS true "

Please correct me if I am wrong .

I) which of the following MUST be true

II) which of the following is ALWAYS true ,

I think for both of these the answer to this question will be different ?

Re: If y=−m^2, which of the following must be true? [#permalink]
02 Jun 2012, 06:26

Expert's post

Joy111 wrote:

manulath wrote:

Joy111 wrote:

If m is positive then too y is negative so statement iii is contradicted ?

In response to the doubt (also on PM)

Joy the question is asking which must always be true

I will go thru the conditions 1 and 2 once again to help you understand the condition 3

Condition 1 says "y is -ve" That means it is saying that "y=-m^2" is always -ve But if m=0 then y becomes 0. So condition 1 is not true

We rule out choices A and D

Condition 2 says "m is non-negative" That means it is saying that in "y=-m^2" m is either 0 or +ve But it is not true, m can be -ve also From the question stem, there is no restriction on value of m, and hence m can be any number

We rule out choices B and E

solved Answer is C

Pursuing further Codition 3: "If m is negative then y is negative." This condition limits the values of m It is saying that IF m<0 then y is always -ve The only time when y is not -ve is when m=0 as the condition says that m is less than 0 (that is to say that m is not 0)

Hence when m is -ve (less than 0) y is -ve Always True

I think both of you are correct, I was thinking the question to be " Which of the following is ALWAYS true "

Please correct me if I am wrong .

I) which of the following MUST be true

II) which of the following is ALWAYS true ,

I think for both of these the answer to this question will be different ?

No, they are the same. Option III must be true because it's always true.

The number zero is neither positive nor negative, and therefore has no sign. In arithmetic, +0 and −0 both denote the same number 0, and the negation of zero is zero itself.

I thought it is not allowed to write -0 ,since 0 is neither negative, nor positive. but wiki says -0 is ok and it means just 0. ok, Bunuel, now I agree with u

_________________

Happy are those who dream dreams and are ready to pay the price to make them come true

Re: If y=−m^2, which of the following must be true? [#permalink]
19 Jun 2013, 07:00

1

This post received KUDOS

Expert's post

Joy111 wrote:

If y=−m^2, which of the following must be true?

I. y is negative. II. m is non-negative. III. If m is negative then y is negative.

A. I only B. II only C. III only D. I and II only E. II and III only

Given that y = -m^2

If m=0, then y =0 --> This eliminates option I. Again, m can be a non-negative value as there is no restriction on that, thus statement II gets eliminated.

Multiply both sides by m assuming m\neq{0}. Thus, my = -m^3. Now, if m<0,then m^3<0and -m^3>0. Hence, as my>0, m and y must have the same sign.

Re: If y=−m^2, which of the following must be true? [#permalink]
30 May 2014, 12:26

The biggest issue here is obviously noticing that m could be 0, thus making y=0 as well. The first option is a tricky one, but normally the other options will at least offer a hint and/or insight into the trick. In this case, option II mentions "non-negative," which suggests that the reader considers that m could be 0.

Summary: Read all of the option choices! They're trying to give you hints

gmatclubot

Re: If y=−m^2, which of the following must be true?
[#permalink]
30 May 2014, 12:26