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What does x =? Helpp!

This topic has 1 expert reply and 4 member replies
gmatpup Senior | Next Rank: 100 Posts Default Avatar
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What does x =? Helpp!

Post Mon Nov 21, 2011 4:46 am
If x(2x+1)=0 and (x + 1/2)(2x - 3)=0, then x =?

A. -3
B. -1/2
C. 0
D. 1/2
E. 3/2

Answer is B

Please explain how to do this type of question. Thank you Smile!

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rooster Senior | Next Rank: 100 Posts Default Avatar
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Post Mon Nov 21, 2011 11:07 pm
Two ways other than plugging in.

1. Solve for zero:

x(2x+1)=0 and (x + 1/2)(2x - 3)=0, then x =?

In the first equation, you know x= 0 or 2x+1=0, x=-1/2

In the second, in order for it to be zero, x needs to be -1/2 or 3/2.

In order for a term to satisfy both equations, x=-1/2.


OR

Since you know the two are the both equal to zero, you can set them to each other

x(2x+1)=0 and (x + 1/2)(2x - 3)=0, then x =?

x(2x+1)=(x + 1/2)(2x - 3)

2x^2 + x = 2x^2 -3x + x - 3/2

x = -3x + x - 3/2

3x = -3/2

x= -1/2

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bookjive Newbie | Next Rank: 10 Posts Default Avatar
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Post Thu Jan 05, 2012 8:40 pm
How come you don't use the quadratic equation idea on this problem and FOIL? Is it just too slow?

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rooster Senior | Next Rank: 100 Posts Default Avatar
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Post Mon Nov 21, 2011 11:07 pm
Two ways other than plugging in.

1. Solve for zero:

x(2x+1)=0 and (x + 1/2)(2x - 3)=0, then x =?

In the first equation, you know x= 0 or 2x+1=0, x=-1/2

In the second, in order for it to be zero, x needs to be -1/2 or 3/2.

In order for a term to satisfy both equations, x=-1/2.


OR

Since you know the two are the both equal to zero, you can set them to each other

x(2x+1)=0 and (x + 1/2)(2x - 3)=0, then x =?

x(2x+1)=(x + 1/2)(2x - 3)

2x^2 + x = 2x^2 -3x + x - 3/2

x = -3x + x - 3/2

3x = -3/2

x= -1/2

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bookjive Newbie | Next Rank: 10 Posts Default Avatar
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Post Thu Jan 05, 2012 8:40 pm
How come you don't use the quadratic equation idea on this problem and FOIL? Is it just too slow?

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neelgandham Community Manager
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Post Mon Nov 21, 2011 4:28 pm
gmatpup wrote:
If x(2x+1)=0 and (x + 1/2)(2x - 3)=0, then x =?

A. -3
B. -1/2
C. 0
D. 1/2
E. 3/2

Answer is B
Please explain how to do this type of question. Thank you Smile!
gmatpup,

Let us solve for x here
x(2x+1)=0 => x = 0 or 2x+1 = 0 => x = 0 or x = -1/2
(x + 1/2)(2x - 3)=0 => x + 1/2 = 0 or 2x - 3 = 0 => x =-1/2 or x =3/2

x=-1/2 satisfies both the equations Option
Quote:
B

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