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# M28-13

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Math Expert
Joined: 02 Sep 2009
Posts: 94616
Own Kudos [?]: 643834 [17]
Given Kudos: 86753
Math Expert
Joined: 02 Sep 2009
Posts: 94616
Own Kudos [?]: 643834 [4]
Given Kudos: 86753
Intern
Joined: 16 Sep 2013
Posts: 1
Own Kudos [?]: [0]
Given Kudos: 11
Concentration: Accounting, Finance
GMAT 1: 510 Q29 V21
GMAT 2: 460 Q33 V19
GPA: 3.95
Math Expert
Joined: 02 Sep 2009
Posts: 94616
Own Kudos [?]: 643834 [0]
Given Kudos: 86753
Viktorijakm wrote:
I think this question is poor.
it is 29 in the question ,but factoring shows 25!!!

You are wrong.

Did you try to expand (x^2 - 25)(x^2 - 4) = 0? You'll get x^4 - 29x^2 + 100 = 0, which is the same as x^4 = 29x^2 - 100.
Intern
Joined: 21 May 2015
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I think this is a high-quality question and I agree with explanation.
Director
Joined: 04 Jun 2018
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Location: United States
Concentration: General Management, Finance
GMAT 1: 730 Q47 V44
GPA: 3.4
Viktorijakm wrote:
I think this question is poor.
it is 29 in the question ,but factoring shows 25!!!

The factoring is correct though, as you are looking for numbers which add to -29 and multiply to 100 respectively.

I think the question is of high quality and similar to stuff I have seen on the GMAC prep tests.
Intern
Joined: 23 May 2017
Posts: 1
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Location: India
Schools: ISB '20 (I)
GMAT 1: 710 Q47 V41
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I think this is a high-quality question.
Current Student
Joined: 01 Jun 2020
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Great question

An alternative approach:

$$x^4 - 29x^2 + 100 = 0$$

Create a variable $$y = x^2$$
$$x^4 - 29x^2 + 100$$
$$y^2-29y^2+100 = 0$$

y = 25 or y = 4

Substitute the values of y in the equation $$y = x^2$$
$$x = y^2$$

Find X:
If y = 25 then x = 5 or x = -5
If y = 4 then x = 2 or x = -2
Math Expert
Joined: 02 Sep 2009
Posts: 94616
Own Kudos [?]: 643834 [0]
Given Kudos: 86753