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Re: What does |2b| equal? [#permalink]
25 Jun 2012, 11:55

4

This post received KUDOS

Expert's post

What does |2b| equal?

(1) b^2-|b|-20=0. Solve quadratics for |b|: (|b|)^2-|b|-20=0 --> |b|=-4 or |b|=5. Since absolute value cannot be negative then we have that |b|=5 and |2b|=10. Sufficient.

(2) |2b|=3b+25. Two cases:

If b\leq{0} then we would have that -2b=3b+25 --> b=-5. If b>0 then we would have that 2b=3b+25 --> b=-25, but since we are considering the rangeo when b>{0} then discard this solution.

Re: What does |2b| equal? [#permalink]
26 Jun 2012, 07:09

Hey Bunuel, the original question contained b^2-|b|-20=0 but you considered it to |b|^2-|b|-20=0 I know it does not matter that if it is |b|^2 or b^2 But it does matter especially when it is not mentioned that b is positive number...
_________________

Re: What does |2b| equal? [#permalink]
26 Jun 2012, 07:13

Expert's post

manimani wrote:

Hey Bunuel, the original question contained b^2-|b|-20=0 but you considered it to |b|^2-|b|-20=0 I know it does not matter that if it is |b|^2 or b^2 But it does matter especially when it is not mentioned that b is positive number...

I'm not sure I understand your question. Anyway:

We are asked to find the value of |2b|, so we don't really care for (1) whether b is positive or negative.
_________________

Re: What does |2b| equal? [#permalink]
26 Jun 2012, 08:03

Bunuel wrote:

What does |2b| equal?

(1) b^2-|b|-20=0. Solve quadratics for |b|: (|b|)^2-|b|-20=0 --> |b|=-4 or |b|=5. Since absolute value cannot be negative then we have that |b|=5 and |2b|=10. Sufficient.

(2) |2b|=3b+25. Two cases: If b\leq{0} then we would have that -2b=3b+25 --> b=-5. If b>0 then we would have that 2b=3b+25 --> b=-25, but since we are considering the rangeo when b>{0} then discard this solution.

So, we have that b=-5, hence |2b|=10. Sufficient.

Answer: D.

Hope it's clear.

Hi, can you make the solution more clear?in case of (1) why you didn't consider the negative value of b? When b is positive, b^2 -b = 20 and when b is negative b^2 + b =20..then we will find 4 values of b...I am confused ..Can you explain? and in case of (2) how you discard the 2nd value? Thanks

Re: What does |2b| equal? [#permalink]
26 Jun 2012, 08:21

1

This post received KUDOS

Expert's post

farukqmul wrote:

Bunuel wrote:

What does |2b| equal?

(1) b^2-|b|-20=0. Solve quadratics for |b|: (|b|)^2-|b|-20=0 --> |b|=-4 or |b|=5. Since absolute value cannot be negative then we have that |b|=5 and |2b|=10. Sufficient.

(2) |2b|=3b+25. Two cases: If b\leq{0} then we would have that -2b=3b+25 --> b=-5. If b>0 then we would have that 2b=3b+25 --> b=-25, but since we are considering the rangeo when b>{0} then discard this solution.

So, we have that b=-5, hence |2b|=10. Sufficient.

Answer: D.

Hope it's clear.

Hi, can you make the solution more clear?in case of (1) why you didn't consider the negative value of b? When b is positive, b^2 -b = 20 and when b is negative b^2 + b =20..then we will find 4 values of b...I am confused ..Can you explain? and in case of (2) how you discard the 2nd value? Thanks

OK.

Say x=|b|, then we have that x^2-x-20=0 --> x=-4 or x=5. Now, x=|b|=-4 is not possible, since an absolute value of a number (|b|) cannot be negative. So, we have that |b|=5 and |2b|=10.

Now, you can solve this statement considering two ranges: b\leq{0} and b>0, which will lead you to the same.

If b\leq{0} then we'll have b^2+b-20=0 --> b=-5 or b=4 (not a valid solution since we are considering the range when b\leq{0}); If b>{0} then we'll have b^2-b-20=0 --> b=-4 (not a valid solution since we are considering the range when b>{0}) or b=5;

So, only two valid solutions: b=-5 or b=5 --> |2b|=10.

As for (2), it's explained in the post: If b>0 then we would have that 2b=3b+25 --> b=-25, but since we are considering the rangeo whenb>{0} then discard this solution.

Re: What does |2b| equal? [#permalink]
26 Jun 2012, 08:22

2

This post received KUDOS

farukqmul wrote:

Bunuel wrote:

What does |2b| equal?

(1) b^2-|b|-20=0. Solve quadratics for |b|: (|b|)^2-|b|-20=0 --> |b|=-4 or |b|=5. Since absolute value cannot be negative then we have that |b|=5 and |2b|=10. Sufficient.

(2) |2b|=3b+25. Two cases: If b\leq{0} then we would have that -2b=3b+25 --> b=-5. If b>0 then we would have that 2b=3b+25 --> b=-25, but since we are considering the rangeo when b>{0} then discard this solution.

So, we have that b=-5, hence |2b|=10. Sufficient.

Answer: D.

Hope it's clear.

Hi, can you make the solution more clear?in case of (1) why you didn't consider the negative value of b? When b is positive, b^2 -b = 20 and when b is negative b^2 + b =20..then we will find 4 values of b...I am confused ..Can you explain? and in case of (2) how you discard the 2nd value? Thanks

hey farukqmul I also had the same doubt but then i tried to solve it properly and i found out that there is no need to do it the long way but i will post here to clear your doubt b^2-|b|-20=0 Considering b>0 eq. becomes b^2-b-20=0 which gives us solution b=-4 and 5 since we assumed b>0 so b=-4 is rejected Considering b<0 eq. becomes b^2+b-20=0 which gives us solution b=4 and -5 since we assumed b<0 so b=4 is rejected In both the cases |2b|=10 Now for the 2nd part (copying Buenel's explanation) |2b|=3b+25. Two cases: If b>0 then we would have that -2b=3b+25 --> b=-5. If b>0 then we would have that 2b=3b+25 --> b=-25, but since we are considering b>0 then discard this solution. Hope it's all clear now.
_________________

Re: What does |2b| equal? [#permalink]
27 Jul 2012, 02:32

Hi, I am wondering why |b| cannot be other numbers greater than 5. What if |b| was to equal 16? This would also allow us to solve the quadratics right? in this case (1) would have multiple answers, 5 and 6, therefore causing (1) to be insufficient?

Please let me know if I misunderstand something. Thank you

Re: What does |2b| equal? [#permalink]
27 Jul 2012, 03:20

Solving the equation x^2 - x - 20 = 0 gives you two possible solution which are -4 and +5 In our case x = |b| so x cannot be negative. That let x = 5 the only possible solution

Re: What does |2b| equal? [#permalink]
27 Jul 2012, 06:43

hi Arthur, I understand your point. However, I do not understand why "16" cannot be another possible answer. Since if we allow |b| to be 16, we can also solve the quadratics.

Re: What does |2b| equal? [#permalink]
27 Jul 2012, 07:20

Expert's post

naruphanp wrote:

hi Arthur, I understand your point. However, I do not understand why "16" cannot be another possible answer. Since if we allow |b| to be 16, we can also solve the quadratics.

Frankly the above does not make ANY sense. How is |b|=16 satisfy any of the statements?
_________________

Re: What does |2b| equal? [#permalink]
28 Jul 2012, 17:51

Bunuel wrote:

What does |2b| equal?

(1) b^2-|b|-20=0. Solve quadratics for |b|: (|b|)^2-|b|-20=0 --> |b|=-4 or |b|=5. Since absolute value cannot be negative then we have that |b|=5 and |2b|=10. Sufficient.

(2) |2b|=3b+25. Two cases: If b\leq{0} then we would have that -2b=3b+25 --> b=-5. If b>0 then we would have that 2b=3b+25 --> b=-25, but since we are considering the rangeo when b>{0} then discard this solution.

So, we have that b=-5, hence |2b|=10. Sufficient.

Answer: D.

Hope it's clear.

Hey Bunuel,

I have always been meaning to ask you. I know that (1) |x|=x if x>0 and (2) |x|=-x if x <0.

But the "=" of the (less than or equal to) or (greater than or equal to), where does it go? I noticed that you put it on equation 2, while others place it on equation (1). Its Driving me nuts!!

Re: What does |2b| equal? [#permalink]
29 Jul 2012, 00:50

Expert's post

alphabeta1234 wrote:

Bunuel wrote:

What does |2b| equal?

(1) b^2-|b|-20=0. Solve quadratics for |b|: (|b|)^2-|b|-20=0 --> |b|=-4 or |b|=5. Since absolute value cannot be negative then we have that |b|=5 and |2b|=10. Sufficient.

(2) |2b|=3b+25. Two cases: If b\leq{0} then we would have that -2b=3b+25 --> b=-5. If b>0 then we would have that 2b=3b+25 --> b=-25, but since we are considering the rangeo when b>{0} then discard this solution.

So, we have that b=-5, hence |2b|=10. Sufficient.

Answer: D.

Hope it's clear.

Hey Bunuel,

I have always been meaning to ask you. I know that (1) |x|=x if x>0 and (2) |x|=-x if x <0.

But the "=" of the (less than or equal to) or (greater than or equal to), where does it go? I noticed that you put it on equation 2, while others place it on equation (1). Its Driving me nuts!!

First of all: |x|=-x when x\leq{0};

|x|=x when x\geq{0}.

Next, as for "=" sign in the solution: you could include it either in the first case or in the second, it doesn't matter at all.
_________________

Re: What does |2b| equal? [#permalink]
04 Jul 2013, 20:49

1

This post received KUDOS

(1) If b >0, eqn reduces to (b-5)(b+4)=0 , since we have assumed b >0, this gives us a soln of b=5. If b <0, eqn reduces to (b+5)(b-4) =0 giving us a soln of b=-5 (since b <0). Since we are asked for the value of |2b| it doesnt matter if b is 5 or -5. Sufficient.

(2) If b >0, it gives us 2b=3b+25 or b =-25 but tht goes against our assumption. So b <0 and -2b=3b+25 or b=-5. Sufficient.

Re: What does |2b| equal? [#permalink]
07 Jul 2013, 17:24

What does |2b| equal?

(1) b^2-|b|-20=0 use quadratic formula to try and get a value for b. (|b|-5) (|b|+4) = 0 |b|=5 OR |b|=-4 Well, an absolute value cannot be equal to a negative number so |b|=5. Therefore, |2b|=10 SUFFICIENT

(2) |2b|=3b+25

Find positive/negative cases to try and isolate b. |2b|=3b+25 If x>= 0 2b=3b+25 -b=25 b=-25 Not valid as -2b is not greater than or equal to zero. x<0 -2b=3b+25 -5b=25 b=-5 Valid as -5 falls within the range of <0 SUFFICIENT (D)

gmatclubot

Re: What does |2b| equal?
[#permalink]
07 Jul 2013, 17:24