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(3x - 7y)^2

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(3x - 7y)^2 [#permalink] New post 05 Nov 2017, 01:12
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Question Stats:

89% (00:20) correct 10% (00:22) wrong based on 39 sessions


Quantity A
Quantity B
\((3x - 7y)^2\)
\(9x^2 - 42xy + 49y^2\)


A) Quantity A is greater.
B) Quantity B is greater.
C) The two quantities are equal.
D) The relationship cannot be determined from the information given.
[Reveal] Spoiler: OA

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Re: (3x - 7y)^2 [#permalink] New post 05 Nov 2017, 02:55
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Quantity A is the square of a binomial that is computed as \((a-b)^2 = a^2-2ab+b^2\). Thus, applying the formula to our case, we get exactly the same quantity of B.

Answer C
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Re: (3x - 7y)^2 [#permalink] New post 28 Mar 2018, 15:01
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Answer: C
(3x - 7y)^2 = (3x)^2 + (7y)^2 - 2*3x*(-7y) = 9x^2 + 49y^2 - 42xy
These two are equal.
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Re: (3x - 7y)^2 [#permalink] New post 29 Mar 2018, 13:42
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Carcass wrote:


Quantity A
Quantity B
\((3x - 7y)^2\)
\(9x^2 - 42xy + 49y^2\)


A) Quantity A is greater.
B) Quantity B is greater.
C) The two quantities are equal.
D) The relationship cannot be determined from the information given.


Ideally you should use the foil method but since time is an issue with the GRE you should probably just cheat and take a shortcut.

9x^2 - 42xy + 49y^2 is a perfect trinomial so we can use a special formula to simplify it into a binomial.

The first term will be the square root of the trinomial's first term (9x^2) which is 3x.
The second term will be the suare root of the trinomials term term (49y^2) which is 7y.
A negative sign with a positive sign gets us a negative sign.

Our binomial is hence 3x-7y.

Notice that if you multiply 3x by -7y you will get -21xy which is half of -42xy.

Answer is C.
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Re: (3x - 7y)^2 [#permalink] New post 16 May 2018, 03:33
Will it be a wrong approach to plug in different values of X and Y and land on the option D.

however, certainly the answer can be found by expanding the formula.
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Re: (3x - 7y)^2 [#permalink] New post 16 May 2018, 04:07
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IshanGre wrote:
Will it be a wrong approach to plug in different values of X and Y and land on the option D.

however, certainly the answer can be found by expanding the formula.



Plugging values is a method of elemination. Ideally one should use it when other options are not available.

Also ideally pick questions where the number of variables is low. Otherwise the number of values you need totest your hypothesis becomes too large.

For example a question with one variable can be tested with 2 or 3 points

A question with 2 variables would need 4 or 9 points (2points * 2points (4) or 3 points * 3points (9)).

In this case best avoid plugging unless you have run out of options.
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Re: (3x - 7y)^2 [#permalink] New post 17 May 2018, 21:44
sandy wrote:
IshanGre wrote:
Will it be a wrong approach to plug in different values of X and Y and land on the option D.

however, certainly the answer can be found by expanding the formula.



Plugging values is a method of elemination. Ideally one should use it when other options are not available.

Also ideally pick questions where the number of variables is low. Otherwise the number of values you need totest your hypothesis becomes too large.

For example a question with one variable can be tested with 2 or 3 points

A question with 2 variables would need 4 or 9 points (2points * 2points (4) or 3 points * 3points (9)).

In this case best avoid plugging unless you have run out of options.


Thats great advice!! thank you-- -I got your point , i'll plug in values when the variables numbers are low.
Re: (3x - 7y)^2   [#permalink] 17 May 2018, 21:44
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