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Co-ordinate Geometry

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by dhairya275 » Sun Sep 16, 2012 12:20 pm
The equation of line n is y = (4/3)x- 100. What is the smallest possible distance in the xy-plane from the point with coordinates (0, 0) to any point on line n?

a)48
b)50
c)60
d)75
e)100

I'll post the answer later. Thanks
Last edited by dhairya275 on Mon Sep 17, 2012 12:00 am, edited 1 time in total.
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Source: — Data Sufficiency |

by GMATGuruNY » Sun Sep 16, 2012 2:20 pm
dhairya275 wrote:The equation of line n is y = (4/3)x - 100. What is the smallest possible distance in the xy-plane from the point with coordinates (0, 0) to any point on line n?

a)48
b)50
c)60
d)75
e)100

I'll post the answer later. Thanks
I've corrected a typo: the equation of the line should be y = (4/3)x - 100.

The x-intercept of a line occurs when y=0.
Substituting y=0 into y = (4/3)x - 100, we get:
0 = (4/3)x - 100
x = 75.
Thus, the x-intercept of y = (4/3)x - 100 is (75,0).

DRAW the figure:
Image

The SHORTEST DISTANCE between a point and a line must form a RIGHT ANGLE with the line.
Thus:
d = the shortest distance between the origin and y = (4/3)x - 100.

∆ABC is a multiple of a 3-4-5 triangle:
25*(3:4:5) = 75:100:125.
Thus, BC = 125.

Any side of a triangle can be considered the base.
Each base has a corresponding height.
A = 1/2(bh).
Since the area must be the same no matter which base and height are used, bh must always yield the same product.
In the triangle above:
If AB=75 is considered the base, the corresponding height is AC=100.
If BC=125 is considered the base, the corresponding height is d.
Since bh must yield the same product in each case, we get:
75*100 = 125d
d = 60.

The correct answer is C.
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by coolhabhi » Mon Sep 17, 2012 11:46 am
If we have the equation Ax + By + C = 0 and the point as (x1, y1) then the PERPENDICULAR DISTANCE onto the line from the point (x1, y1) is :

Image

So I transformed the equation as 4x - 3y - 300 = 0 and the point is (0, 0).
So the perpendicular distance is
|4(0) - 3(0) - 300|
------------------------
√ (3^2 + 4^2)

|300|
--------
5

Answer : 60
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