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Re: Need help in Coordinate geometry [#permalink]
11 Jun 2010, 07:11

C

equation of a line y = mx+c. If y intercept of line l is 4, then one of the points is (0,4) - by making x zero.

We know that the slope 'm' is negative. To find the x intercept of the line, make y zero in the above line equation. We get x = -c/m. Since the value of c is 4 and the slope is negative. x has to be positive, and not negative or 0.

Re: The y intercept of line l is 4. If the slope of line l is [#permalink]
22 Jul 2012, 18:48

Hi Bunuel, I am not convinced with the answer for the above question. Let me tell you how I approached it. The equation is y=mx+4, where 4 is the y-intercept (given) and since it has got a negative slope, the equation becomes, y=(-m)x+4. Now, to determine x-intercept, we consider y=0, then, -mx+4=0, hence mx=4 and since m has to be negative (it is given that slope is negative for the line), I chose x=-1, so that m=-4. According to the above calculations, I get the answer as A. But the answer is C. Please help me get the concept right.

Re: The y intercept of line l is 4. If the slope of line l is [#permalink]
23 Jul 2012, 00:47

The answer is C. If the slope is negative of the line then either it passes through the 1 st quadrant <starting from II towards IV> or passes through the third quadrant <starting from II towards IV>.. Thus, the intercepts are either both positive or both negative. Given in the question, its a positive intercept, the other one will be a positive as well. Thus, 6.

Re: The y intercept of line l is 4. If the slope of line l is [#permalink]
23 Jul 2012, 02:16

1

This post received KUDOS

sharmila79 wrote:

Hi Bunuel, I am not convinced with the answer for the above question. Let me tell you how I approached it. The equation is y=mx+4, where 4 is the y-intercept (given) and since it has got a negative slope, the equation becomes, y=(-m)x+4. Now, to determine x-intercept, we consider y=0, then, -mx+4=0, hence mx=4 and since m has to be negative (it is given that slope is negative for the line), I chose x=-1, so that m=-4. According to the above calculations, I get the answer as A. But the answer is C. Please help me get the concept right.

No. You are mixing it up. the slope is negative Hence you can write y=(-m)x+4 since -m is negative, in this case m must be positive again your slope is -m, not m!!

So later if you say mx=4 x is necessarily positive since m is positive.

Re: The y intercept of line l is 4. If the slope of line l is [#permalink]
23 Jul 2012, 02:18

Also, even though the answer is C, this problem is not very well written IMO since an infinite negative slope could cause the x intercept to be 0 :D (though "The intercept" can imply that there is only one y intercept)

gmatclubot

Re: The y intercept of line l is 4. If the slope of line l is
[#permalink]
23 Jul 2012, 02:18