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Bunuel
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If [3(ab)^3 + 9(ab)^2 54ab] / [(a-1)(a + 2)] = 0, and a and b are bo 05 Nov 2014, 08:46
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If \(\frac{3(ab)^3 + 9(ab)^2 - 54ab}{(a-1)(a + 2)} = 0\), and a and b are both non-zero integers, which of the following could be the value of b?

I. 2
II. 3
III. 4

(A) I only
(B) II only
(C) I and II only
(D) I and III only
(E) I, II, and III
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A
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PareshGmat
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If [3(ab)^3 + 9(ab)^2 54ab] / [(a-1)(a + 2)] = 0, and a and b are bo 09 Nov 2014, 20:45
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\(\frac{3(ab)^3 + 9(ab)^2 - 54ab}{(a-1)(a + 2)} = 0\)

\(3(ab)^3 + 9(ab)^2 - 54ab = 0\)

Divide the complete equation by 3ab

\((ab)^2 + 3(ab) - 18 = 0\)

ab = -6 OR ab = 3

For ab = -6

if b = 2, then a = -3 (Satisfies denominator)

if b = 3, then a = -2 (Does not satisfy denominator)

For ab = 3

if b = 3, then a = 1 (Does not satisfy denominator)

Answer = A
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If [3(ab)^3 + 9(ab)^2 54ab] / [(a-1)(a + 2)] = 0, and a and b are bo 06 Nov 2014, 02:19
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Bunuel

Tough and Tricky questions: Algebra.



If \(\frac{3(ab)^3 + 9(ab)^2 - 54ab}{(a-1)(a + 2)} = 0\), and a and b are both non-zero integers, which of the following could be the value of b?

I. 2
II. 3
III. 4

(A) I only
(B) II only
(C) I and II only
(D) I and III only
(E) I, II, and III

Kudos for a correct solution.
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\(\frac{3(ab)^3 + 9(ab)^2 - 54ab}{(a-1)(a + 2)} = 0\)

we cannot have a=1, and a=-2. because this will make denominator zero.

now, consider the numerator. let ab=k
thus we have k(3k^2 +9k-54)
=k(3k^2+18k-9k-54)
=k(3k(k+6)-9(k+6))
=k(3k-9)(k+6)

if 3k-9=0
3k=9
k=3
i.e. ab=3
now if b=3, then a=1, which is not acceptable as a=1, will make the denominator equal to zero.
if k+6=0
k=-6
or ab=-6

again, if b=3, then a=-2, which is not acceptable as a=-2, will make the denominator equal to zero.

thus except b=3, all other values could be possible. hence answer should be D
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If [3(ab)^3 + 9(ab)^2 54ab] / [(a-1)(a + 2)] = 0, and a and b are bo 06 Nov 2014, 11:43
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manpreetsingh86
Bunuel

Tough and Tricky questions: Algebra.



If \(\frac{3(ab)^3 + 9(ab)^2 - 54ab}{(a-1)(a + 2)} = 0\), and a and b are both non-zero integers, which of the following could be the value of b?

I. 2
II. 3
III. 4

(A) I only
(B) II only
(C) I and II only
(D) I and III only
(E) I, II, and III

Kudos for a correct solution.

\(\frac{3(ab)^3 + 9(ab)^2 - 54ab}{(a-1)(a + 2)} = 0\)

we cannot have a=1, and a=-2. because this will make denominator zero.

now, consider the numerator. let ab=k
thus we have k(3k^2 +9k-54)
=k(3k^2+18k-9k-54)
=k(3k(k+6)-9(k+6))
=k(3k-9)(k+6)

if 3k-9=0
3k=9
k=3
i.e. ab=3
now if b=3, then a=1, which is not acceptable as a=1, will make the denominator equal to zero.
if k+6=0
k=-6
or ab=-6

again, if b=3, then a=-2, which is not acceptable as a=-2, will make the denominator equal to zero.

thus except b=3, all other values could be possible. hence answer should be D
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As ab=-6 or ab=3, so if we consider value of b as 4, a becomes Fraction in both the cases, but a is a non-zero integer(Given in the question). Hence b cannot be 4, so the answer should be (A) I only
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If [3(ab)^3 + 9(ab)^2 54ab] / [(a-1)(a + 2)] = 0, and a and b are bo 06 Nov 2014, 11:56
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IMO ans shud be A

using numerator and taking ab= x
we get
3x(x^2+3x-18)=0
gives 2 equations
3x=0 .i.e 3 ab =0 i.e a=0 or b=0 # but questions has given both are non zero so it can be true
therefore
x^2-3x+18=0 , gives
can be factorized as
(x-6)(x+3)=0
gives
x=6 or ab=6 it has soloutions as (1,6),(6,1),(2,3),3,2)
a=1 and a=2 is not possible as questions will become non mathematical.
Therfore solutions possible are (6,1) and (3,2)
Therfore ans is A b=2
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If [3(ab)^3 + 9(ab)^2 54ab] / [(a-1)(a + 2)] = 0, and a and b are bo 06 Nov 2014, 23:37
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manpreetsingh86
Bunuel

Tough and Tricky questions: Algebra.



If \(\frac{3(ab)^3 + 9(ab)^2 - 54ab}{(a-1)(a + 2)} = 0\), and a and b are both non-zero integers, which of the following could be the value of b?

I. 2
II. 3
III. 4

(A) I only
(B) II only
(C) I and II only
(D) I and III only
(E) I, II, and III

Kudos for a correct solution.

\(\frac{3(ab)^3 + 9(ab)^2 - 54ab}{(a-1)(a + 2)} = 0\)

we cannot have a=1, and a=-2. because this will make denominator zero.

now, consider the numerator. let ab=k
thus we have k(3k^2 +9k-54)
=k(3k^2+18k-9k-54)
=k(3k(k+6)-9(k+6))
=k(3k-9)(k+6)

if 3k-9=0
3k=9
k=3
i.e. ab=3
now if b=3, then a=1, which is not acceptable as a=1, will make the denominator equal to zero.
if k+6=0
k=-6
or ab=-6

again, if b=3, then a=-2, which is not acceptable as a=-2, will make the denominator equal to zero.

thus except b=3, all other values could be possible. hence answer should be D
Show more

ab=-6
consider b=4, then a=-3/2 which is not a integer. But according to the question a,b are non zero integer. So, answer is A
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If [3(ab)^3 + 9(ab)^2 54ab] / [(a-1)(a + 2)] = 0, and a and b are bo 10 Nov 2014, 23:46
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I just have this question, why cant be a=1 or -2. Why cant the denominator equal zero..
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If [3(ab)^3 + 9(ab)^2 54ab] / [(a-1)(a + 2)] = 0, and a and b are bo 11 Nov 2014, 03:51
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sunaimshadmani
I just have this question, why cant be a=1 or -2. Why cant the denominator equal zero..
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Division by 0 is not allowed: anything/0 is undefined.
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If [3(ab)^3 + 9(ab)^2 54ab] / [(a-1)(a + 2)] = 0, and a and b are bo 14 Jan 2015, 04:06
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PareshGmat
\(\frac{3(ab)^3 + 9(ab)^2 - 54ab}{(a-1)(a + 2)} = 0\)

\(3(ab)^3 + 9(ab)^2 - 54ab = 0\)

Divide the complete equation by 3ab

\((ab)^2 + 3(ab) - 18 = 0\)

ab = -6 OR ab = 3

For ab = -6

if b = 2, then a = -3 (Satisfies denominator)

if b = 3, then a = -2 (Does not satisfy denominator)

For ab = 3

if b = 3, then a = 1 (Does not satisfy denominator)

Answer = A
Show more

One question here.

For For ab = 3, why cant we have:
ab=3 --> ab=1(3), cannot be
ab=3 --> ab=3(1), couldn't this one also be? Or it is not included in the solution because there is not "1" as an answer option?
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If [3(ab)^3 + 9(ab)^2 54ab] / [(a-1)(a + 2)] = 0, and a and b are bo 08 Jun 2016, 01:37
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Bunuel

Tough and Tricky questions: Algebra.



If \(\frac{3(ab)^3 + 9(ab)^2 - 54ab}{(a-1)(a + 2)} = 0\), and a and b are both non-zero integers, which of the following could be the value of b?

I. 2
II. 3
III. 4

(A) I only
(B) II only
(C) I and II only
(D) I and III only
(E) I, II, and III

Kudos for a correct solution.
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\(\frac{3(ab)^3 + 9(ab)^2 - 54ab}{(a-1)(a + 2)} = 0\)

Take 3ab common in the numerator.

\(\frac{3(ab) * [(ab)^2 + 3(ab) - 18]}{(a-1)(a + 2)} = 0\)

Note that \((ab)^2 + 3ab - 18\) is a quadratic which we can split into factors just like we do for \(x^2 + 3x - 18\).
\(x^2 + 3x - 18 = (x + 6)(x - 3)\) so \((ab)^2 + 3ab - 18 = (ab + 6)*(ab - 3)\)


\(\frac{3(ab) * [(ab + 6)*(ab - 3)]}{(a-1)(a + 2)} = 0\)

Now, when will this fraction be 0? When one of the factors in the numerator is 0. (Note that denominator cannot be 0. So a cannot the 1 or -2.)

So either ab = 0 (a and b are non zero so not possible)
or (ab + 6) = 0 i.e. ab = -6
or (ab - 3) = 0 i.e. ab = 3

So, we get that one of these two must hold. Either ab = -6 or ab = 3.

Now let's look at the possible values of b.
Can b be 2? If b = 2, a = -3 (in this case, ab = -6. Satisfies)
Can b be 3? If b = 3, a is -2 or 1. Both values are not possible so b cannot be 3.
Can b be 4? If b = 4, a is not an integer in either case. So b cannot be 4.

Answer (A)
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If [3(ab)^3 + 9(ab)^2 54ab] / [(a-1)(a + 2)] = 0, and a and b are bo 20 Aug 2018, 10:36
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Why does the 3ab which is factored out not translate to ab=0 as a possible answer?
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dwhitmer21
Why does the 3ab which is factored out not translate to ab=0 as a possible answer?
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The prompt indicates that a and b are NONZERO INTEGERS.
Thus, ab=0 is not a valid case.
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If [3(ab)^3 + 9(ab)^2 54ab] / [(a-1)(a + 2)] = 0, and a and b are bo 17 Nov 2018, 10:21
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Bunuel

Tough and Tricky questions: Algebra.



If \(\frac{3(ab)^3 + 9(ab)^2 - 54ab}{(a-1)(a + 2)} = 0\), and a and b are both non-zero integers, which of the following could be the value of b?

I. 2
II. 3
III. 4

(A) I only
(B) II only
(C) I and II only
(D) I and III only
(E) I, II, and III

Kudos for a correct solution.
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Please tell me where i am going wrong.Bunuel VeritasKarishma chetan2u

3(ab)^3 + 9 (ab)^2=54 ab
3(ab)^2[ab+3]=54 ab
div by 3 ab b/s
ab[ab+3]=18
let ab =z
z[z+3]=18
z[z+3]=3*6
hence, ab=3
so a cant b 1 so b cnt b 3
but m not getting rest of soltuins?i know i some how messed up by not making quadratic equation ,can u please guide me where i m going wrong?
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Bunuel

Tough and Tricky questions: Algebra.



If \(\frac{3(ab)^3 + 9(ab)^2 - 54ab}{(a-1)(a + 2)} = 0\), and a and b are both non-zero integers, which of the following could be the value of b?

I. 2
II. 3
III. 4

(A) I only
(B) II only
(C) I and II only
(D) I and III only
(E) I, II, and III

Kudos for a correct solution.

Please tell me where i am going wrong.Bunuel VeritasKarishma chetan2u

3(ab)^3 + 9 (ab)^2=54 ab
3(ab)^2[ab+3]=54 ab
div by 3 ab b/s
ab[ab+3]=18
let ab =z
z[z+3]=18
z[z+3]=3*6
hence, ab=3
so a cant b 1 so b cnt b 3
but m not getting rest of soltuins?i know i some how messed up by not making quadratic equation ,can u please guide me where i m going wrong?
Show more

You are missing out on other value of z...
z(z+3)=18..
Here z can be -6 too.
-6(-6+3)=-6*-3=18
So ab=-6..
when a is -3, b is 2.... So I is valid..

Now for b as 4, not possible, otherwise a will become fraction.
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