If p and q are distinct lines in the xy coordinate system : GMAT Data Sufficiency (DS)
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# If p and q are distinct lines in the xy coordinate system

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If p and q are distinct lines in the xy coordinate system [#permalink]

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28 Aug 2013, 19:36
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If p and q are distinct lines in the xy coordinate system such that the equation for p is y= ax + b and the equation for q is y= cx + d, is ac = a²?

(1) d = b + 2
(2) For each point (x, y) on p, there is a corresponding point (x, y + k) on q for some constant x.

I read this question in Princeton Review. In answer part, they show this:
ac = a² so a = c. But if a = 2 and c = -2, for example? I can't get it.
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If p and q are distinct lines in the xy coordinate system [#permalink]

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22 Aug 2014, 06:08
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leekaru14 wrote:
If p and q are distinct lines in the xy coordinate system such that the equation for p is y= ax + b and the equation for q is y= cx + d, is ac = a²?

(1) d = b + 2
(2) For each point (x, y) on p, there is a corresponding point (x, y + k) on q for some constant x.

I read this question in Princeton Review. In answer part, they show this:
ac = a² so a = c. But if a = 2 and c = -2, for example? I can't get it.

I think statement 2 should read " for some constant k"

is ac = a²?
or is $$ac-a^2= 0$$ --> a(c-a)= 0 --> either a = 0 or a = c
so we need to know if both the lines have same slope or is the slope of line p = 0

(1) d = b + 2 -> nothing about the slopes , just says line q has y intercept which is 2 greater than the y intercept of p, they could still be parallel and have the same slope or not be parallel and differ in slope.

also line y could have 0 slope or not , Hence insufficient

2) For each point (x, y) on p, there is a corresponding point (x, y + k) on q for some constant k.

let 2 points on P be (2, 3) and (5, 7 ) ( we can take any 2 points )

for each of these 2 points on P there will be 2 points on q such that ( 2, 5) and ( 5, 9 ) here taking k as 2
( x coordinates of points on q remains same as those points in line P and Y co ordinates are increased by a constant k )

now slope is (y2- y1)/ (x2- x1)
if we find the slopes of line P and q taking points according to statement 2 , as shown above then both lines will have same slope
hence a= c , sufficient

( Obviously k can vary , still both the lines will give same slope )
If p and q are distinct lines in the xy coordinate system   [#permalink] 22 Aug 2014, 06:08
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