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Re: Help needed on intersecting lines in coordinate geometry [#permalink]
11 Jun 2010, 07:01

2

This post received KUDOS

Expert's post

gautamsubrahmanyam wrote:

In the xy-plane ,line k passes through the point (1,1) and line m passes through the point (1,-1).Are the lines k and m perpendicular to each other

(1) Lines k and m intersect at the point (1,-1) (2) Line k intersects the x axis at the point (1,0)

Please help on this DS problem

For one line to be perpendicular to another, their slopes must be negative reciprocals of each other (if slope of one line is m than the slope of the line perpendicular to this line is -\frac{1}{m}). In other words, the two lines are perpendicular if and only the product of their slopes is -1.

So basically the question is can we somehow calculate the slopes of these lines.

From stem we have one point for each line.

(1) gives us the second point of line k, hence we can get the slope of this line, but we still know only one point of line m. Not sufficient.

(2) again gives the second point of line k, hence we can get the slope of this line, but we still know only one point of line m. Not sufficient.

(1)+(2) we can derive the slope of line k but for line m we still have only one point, hence we can not calculate its slope. Not sufficient.

Answer: E.

For more on this issue please check Coordinate Geometry chapter of Math Book (link in my signature).

Thanks to everyone that responded to my previous round of posts, it was extremely helpful! I have some more questions that I need help with. Here it goes:

In the xy-plane, line k passes through the point (1,1) and line m passes through the point (1,-1). Are lines ka nd m perpendicular to each other?

(1) Lines k and m intersect at the point (1, -1) (2) Line k intersects the x-axis at the point (1,0)

1. k goes through 1,1 and 1,-1, m can be anyslope. not suff 2. k intersects xaxis at 1,0 <- this probably can be seen from (1) itself. no info about m. not suff 1+2 -> m can be anything. can't say, E
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Thanks for the post but I'm still not grasping the concept. When two lines intersect, doesn't that mean that they are perpendicular to each other?

Yes, when two lines intersect, their angle may or may not be 90 degree. Also we need to find the slope of the two lines (m1 and m2) and for the two lines to be perpendicular, m1 * m2 should be equal to -1.

Since the slope could not be uniquely identified, we cannot conclude that they are perpendicular.
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Re: Help needed on intersecting lines in coordinate geometry [#permalink]
16 May 2012, 08:17

Expert's post

venmic wrote:

I did not understand why you said you can find out only line K slope from statement 1 - why not m

sorry but not able to follow please help

Even when considering the statements together we still know only one point of line m: (1, -1). We cannot get the slope based on just one point of a line.

Re: In the XY plane.. [#permalink]
07 Jun 2012, 15:46

Expert's post

Val1986 wrote:

In the xy-plane, line passes through the point (1,1) and line m passes through the point (1,-1). Are lines k and m perpendicular?

1) Lines k and m intersect at the point (1,-1) 2) Line k intersects the x-axis at the point (1,0)

Merging similar topics.

Val1986 wrote:

Could you tell me how to determine whether the lines are perpendicular just in general??

For one line to be perpendicular to another, their slopes must be negative reciprocals of each other (if slope of one line is m than the slope of the line perpendicular to this line is -\frac{1}{m}). In other words, the two lines are perpendicular if and only the product of their slopes is -1.