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# y=mx+b, Does the line pass thru the negative x axis? 1. m+b

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y=mx+b, Does the line pass thru the negative x axis? 1. m+b [#permalink]

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03 Jun 2007, 14:50
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y=mx+b, Does the line pass thru the negative x axis?

1. m+b > 1

2. mb > 0

Kudos [?]: 14 [0], given: 0

Director
Joined: 26 Feb 2006
Posts: 900

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03 Jun 2007, 17:55
baer wrote:
y=mx+b, Does the line pass thru the negative x axis?
1. m + b > 1
2. mb > 0

we need m and b to find whether the line pass through -ve x axis.

from i: either m and b both +ve or one is +ve and the other one is -ve. also the positive is grater than the absolute value of the -ve one. so not suuf cuz we do not know which is +ve and which is -ve.nsf./

from ii: eithe both -ve or both +ve. nsf/.

from i and ii: both are +ve i,e. the slope is +ve and y intercept is also +ve as well.

so C.

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Senior Manager
Joined: 11 Jun 2006
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03 Jun 2007, 18:29
I'm thinking it's B.

I'm testing plugging in values where m and b are both negative or positive to y=mx+b and I can't come up with a situation where there isn't a negative x intercept.

y=mx+b
0=-1(-1)+-1

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Manager
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03 Jun 2007, 18:59

Since m*c>0
=> m and C are +ve or m and C are -ve in both cases the line has to passes through -ve X axis. Hence sufficient condition.

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Intern
Joined: 23 Apr 2007
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04 Jun 2007, 11:07
Here we have to find that whether line intersect at negative X-axis this means y=0 => x=-b/m

(1) says m+b>1 means atleast one of them is positive so answer can't be exactly determined
(2) mb>0 this means either both are positive or negative so again answer can't be exactly detemined

combining both these statements we can get the ansewr.

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correct way   [#permalink] 04 Jun 2007, 11:07
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