If a = b, and c = d, is |a| = |c|?

Question

(1) |b| = |d|
(2) b = -d

chetan2u · Answer

Hi,
Since it is dealing with modulus, lets see what info we get from the Q..

|a|=|c| means that irreespective of sign of b and d, the answer will be YES, if b and d have SAME NUMERIC VALUE.. 
 lets see the choices..
(1) |b| = |d|
same numeric value..
suff

(2) b = -d
same numeric value..
suff
D

nalinnair · Answer

Statement 1:
|b| = |d|
This implies that either b = d or b = -d  
First consider, b = d
replacing b = a and d = c, we get a = c.
Hence, |a| = |c|
Now, consider, b = -d
Again, replacing b = a and d = c, we get a = -c
Hence, |a| = |c|
Therefore, in either case we are able to answer the question is |a| = |c| = 0? with a definitive 'yes'.
Hence, statement 1 is sufficient. Options B, C and E are ruled out.

Statement 2:
b = -d
Again, replacing b = a and d = c, we get a = -c
Hence, |a| = |c|
Therefore, we are able to answer the question is |a| = |c| = 0? with a definitive 'yes'.
Hence, statement 2 is also sufficient. Option A is also ruled out.

The correct answer, therefore, is  D

MathRevolution · Answer

Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and independent equations ensures a solution.

If a = b, and c = d, is |a| = |c|?

(1) |b| = |d|
(2) b = -d

When you modify the original condition and the question, they become |a|=|c|?--> |b|=|d|?--> b=-d or d?, which makes 1)=2). Also, each of them is yes, which is sufficient.
Therefore, the answer is D.

 Once we modify the original condition and the question according to the variable approach method 1, we can solve approximately 30% of DS questions.

rhine29388 · Answer

From the question it is clear that we have to check if numerical values of a and c are equal (signs will not matter)
Statement 1 says |b| = |d| . It clearly shows that |a| = |c| as a = b and c = d so sufficient
Statement 2 says b = -d. It clearly tells that numerical value of b and d is same which in turn says |a| = |c| so sufficient
Correct answer - D

adkikani · Answer

chetan2u   Bunuel   VeritasPrepKarishma   niks18

I am really confused about opening modulus in both sides of equality 
in spite of going through  chetan2u  signature

I could get away by simply substituting a and c for b and d resp to establish suff for St 1
but St 2 was confusing.

Can you please add your two cents?

niks18 · Answer

Hi  adkikani

mod is always positive, so if  x<0  i.e negative then  |x| =-x

for eg. if  x=-3  then  |-3|=-(-3)=3

so for Statement 2:  b=-d  is simply  |b|=|-d| , apply mod on both sides

which in turn is  |b|=|d| , now substitute the value of  b  &  d  to get  |a|=|c|

Alternatively, we are asked is  |a|=|c| , square both sides to remove mod. so the question becomes Is  a^2=c^2 ?

Statement 2:  b=-d , substitute the value of  b  &  d  to get  a=-c . Now square both sides to get  a^2=c^2 . Sufficient

adkikani · Answer

Hi  niks18

how did you use above knowledge to infer below :

To be more specific,  |b|=|d| , note that we do not know whether b or d is less than zero
but all we know is that b and d have opposite signs. Let me know if I missed anything.

niks18 · Answer

Hi  adkikani

As I mentioned mod is always positive, because it represents the distance from the 0-point in the number line and distance cannot be negative.
so irrespective of the value of b or d; |-d|=|d|

let b=2 then b=-d will result in d=-2

So |2|=2 and |-2|=-(-2)=2=|2| so we have |b|=|-d| =>|b|=|d|

adkikani · Answer

niks18 Crystal clear now. +1 Kudos

AkshdeepS · Answer

Hey niks18,

Red part marked above is inconsistent as to my understanding absolute values are always non-negative rather than always positive.

Please clarify.

QZ

niks18 · Answer

Hi  QZ

You are right that mod is "Always Non negative" i.e. either positive or 0. I think either I missed mentioning 0 or I might had wrote that statement in the context of the question. Anyways thanks for highlighting.

sumi747 · Answer

From statement 1 we get that even if b and d are of opposite signs the absolute value of a would be equal to the absolute value of c. Sufficient.
From statement 1 we get that b and d are of opposite signs, the absolute value of a would be equal to the absolute value of c even then. Sufficient.
IMO D

Posted from my mobile device

kinvestor · Answer

As we our taking absolute values, this seems obvious that irrespective of positive or negative numbers, we will get sufficient statements.

=> |b| = |d|

sufficient.

=> b = -d

Sufficient. D

We can also plug numbers.

Archit3110 · Answer

#1
|b| = |d|
irrespective of b & d sign ; value of a & c will be equal since they are in modulus
sufficient
#2
b=-d
again same logic as #1; sufficient 
IMO D

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If a = b, and c = d, is |a| = |c|?

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