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I've missed this prob. several times now. Arg. Thanks for

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Intern
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Joined: 22 Jul 2008
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I've missed this prob. several times now. Arg. Thanks for [#permalink]

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New post 06 Aug 2008, 19:33
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

I've missed this prob. several times now. Arg.
Thanks for your help!
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Director
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Re: GMATPREP - Slope [#permalink]

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New post 06 Aug 2008, 21:33
y1 = m1x + c1 => 3 = m14 + c1 (Although this substitution is unnecessary)
y2 = m2x + c2 => 3 = m24 + c2 (Although this substitution is unnecessary)
If question said 2 lines intersected, that would have been sufficient to do the question.

Question: is m1*m2 < 0

Statement 1:
x = -c1/m1 (for 1st line)
x = -c2/m2 (for 2nd line)
Given -c1/m1 * -c2/m2 > 0
or c1c2/m1m2 > 0
Anyone of these entity -ve and all others +ve will give satisfy this equation. So no surety that m1*m2<0, therefore A and D out.

Statement 2:
c1*c2<0. This however tells us nothing about m1*m2. Therefore B is also out.

Combining
From 2 we know c1*c2 < 0, so for c1*c2/m1*m2 to be < 0, m1*m2 should be < 0, otherwise statement 1 will not hold.

Question is answered.

So answer C.

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Senior Manager
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Re: GMATPREP - Slope [#permalink]

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New post 06 Aug 2008, 21:57
Eq. for L Y1 = M1X1 + C1
N Y2 = M2X2 + C2
Find the x intercepts; for this take y = 0
x1 = -C1/M1 X2 = -C2/M2

S1. TELLS YOU THAT X1*X2 > 0 (+VE)
-C1/M1 * -C2/M2 > 0
This shows that M1 and M2 can either be both +ve or +ve; hence AD are out

S2. Find the y intercepts by placing x = 0
Tells you that C1*C2 < 0 (-VE)
This tells us nothing about the slopes and hence INSUFF; B is out

Combine: -C1*C2 < 0
For -C1/M1 * -C2/M2 > 0
M1*M2 has to be -ve (negative/negative = positive)

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Re: GMATPREP - Slope [#permalink]

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New post 07 Aug 2008, 18:23
flygmat wrote:
I've missed this prob. several times now. Arg.
Thanks for your help!


line l :
y=mx+c since passes through 4,3=> 3=4m+c
y=m1x+c1 since passes through 4,3=> 3=4m1+c1

(m-m1)4=c1-c ------given

(1) y/c + x/(-c/m)=1
y/c1+x/(-c1/m1)=1

c1c/mm1>0 INSUFFI since mm1>0 or mm1<0

(2)cc1<0 =>INSUFFI no clue about mm1
INSUFFI

(1) and (2) => c1c/mm1>0 and cc1<0 => mm1<0 hence SUFFI
IMO C
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cheers
Its Now Or Never

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Re: GMATPREP - Slope   [#permalink] 07 Aug 2008, 18:23
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