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Retired Moderator G
Joined: 18 Sep 2014
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Coordinate geometry

Mathematics is a wonderful subject where you can solve one problem in numerous ways.
Coordinate geometry simplifies the Geometry much further and makes it easy.

Here is a problem from Veritas question bank simple but wonderful to test various concepts.

Attachment: Geometry_Img48.png [ 7.7 KiB | Viewed 10334 times ]

If the x-coordinate of point E is 4, what is its y-coordinate?

A. $$\frac{-1}{2}$$
B. $$1$$
C. $$\frac{3}{2}$$
D. $$2$$
E. $$\frac{7}{2}$$

Different Methods to solve the above problem

Let me consider

$$A (0,3)$$
$$B (6,0)$$
$$E (4,y)$$
Origin $$O(0,0)$$
Lets drop a perpendicular from E to X axis at $$(4,0)$$ and say it $$F.$$

Distance between any two points $$(x_1,y_1)$$ and $$(x_2,y_2)$$ in coordination plane is

$$\sqrt{(y_2-y_1)^2+(x_2-x_1)^2}$$

$$AE+EB=AB$$

$$\sqrt{4^2+(y-3)^2}$$+$$\sqrt{y^2+2^2}$$=$$\sqrt{6^2+3^2}$$

subtract $$\sqrt{y^2+2^2}$$ on both sides

$$\sqrt{y^2-6y+25}$$=$$\sqrt{45}$$-$$\sqrt{y^2+4}$$

squaring on both sides

$$y^2-6y+25=45+y^2+4-2\sqrt{45}\sqrt{y^2+4}$$

After cancelling similar terms like y^2 and simplifying the terms, Equation looks like

$$3y+12=\sqrt{45*(y^2+4)}$$

squaring on both sides again, we get

$$9(y^2+16+8y)=45(y^2+4)$$

on bringing all terms to one side and simplification, we get $$(y-1)^2=0$$

i.e., $$y=1$$

This method is quite time consuming and cumbersome.

2. Form an equation and Substitute the value

Two Intercept Form for the Equation of a Line

$$\frac{x}{a}+\frac{y}{b}=1$$, where a is the x-intercept and b is the y-intercept.

with points A and B, make an equation for line AB

$$\frac{x}{6}+\frac{y}{3}=1$$

Since point E lies on line AB, substitute $$E(4,y)$$ in above equation

$$\frac{4}{6}+\frac{y}{3}=1$$

solving for y, we get $$y=1$$

Equation of line can also be formed with two points A(x1,y1), B(x2,y2) and slope of line AB m.
It can be represented as

$$m=\frac{(y_2-y_1)}{(x_2-x_1)}$$

equation of line is $$y-y_1=m(x-x_1)$$ or $$y-y_2=m(x-x_2)$$

now we get $$y=1$$ as we substitute $$E(4,y)$$ as above.

3. Midpoint method

Midpoint between (0,3) and (6,0) is (3, (3/2) = (3, 1.5).

Because point E is below the midpoint with an x-coordinate of 4 and the y-coordinate had to be less than 1.5.
The only answer choice below 1.5 yet above 0 is 1 as in option B.

4. Slope method

Equate the slope of Line AE and EB since both the lines Coincide.

$$\frac{(y-3)}{(4-0)}=\frac{(0-y)}{(6-4)}$$

we get solve for y now.

5. Trigonometry

GMAT doesnt require you to know trigonometric functions and values of 0, 30,45,60,90 degree angles but it is always good to know
Those who are not aware of trigonometry, please refer to overview section of below link.
https://en.wikipedia.org/wiki/Trigonometry

Now Lets drop a perpendicular from E to X axis at (4,0) and say it F. It appears as in fig.
Attachment: New Doc 5_1.jpg [ 18.68 KiB | Viewed 10278 times ]

$$tan∠ABO=tan∠EBF$$

$$tan(θ)$$=opposite side/adjacent side

In OAB, $$tan∠ABO=\frac{OA}{OB}$$
In EBF, $$tan∠EBF=\frac{EF}{FB}$$

$$\frac{OA}{OB}$$=$$\frac{EF}{FB}$$

i.e., $$\frac{3}{6}$$=$$\frac{y}{(6-4)}$$

now we can solve for y.

6. Similar triangles

OAB and EFB are similar triangles for EF is parallel to OA.

Triangle Proportionality Theorem.
If a line parallel to one side of a triangle intersects the other two sides of the triangle, then the line divides these two sides proportionally.

$$\frac{BE}{BA}=\frac{BF}{BO}$$=$$\frac{EF}{OA}$$

to solve in simple terms, we take

$$\frac{BF}{BO}$$=$$\frac{EF}{OA}$$

$$\frac{2}{6}=\frac{y}{3}$$ and hence $$y=1$$. Mathematics may not teach us how to add love or how to minus hate. But it gives every reason to hope that every problem has not one but multiple solutions. VP  P
Joined: 12 Dec 2016
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Re: Beauty of Coordinate geometry  [#permalink]

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I have a question, how useful each method is in solving a gmat question?
It is because these methods seem to prove your creativity rather than gmat skills.
Intern  Joined: 09 Nov 2016
Posts: 5
Location: India
Concentration: Marketing, General Management
GPA: 3.6
Re: Beauty of Coordinate geometry  [#permalink]

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I tried finding the slope of line AB by y2-y1/x2-x1 and then formed an equation which came out to be y=-1/2x+3/2 and then substitute x=4 and y=y in the above equation. Got -7/2 as the answer. Can anyone please shed what's wrong with my approach?
VP  P
Joined: 12 Dec 2016
Posts: 1462
Location: United States
GMAT 1: 700 Q49 V33
GPA: 3.64
Re: Beauty of Coordinate geometry  [#permalink]

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1
chavantusharr wrote:
I tried finding the slope of line AB by y2-y1/x2-x1 and then formed an equation which came out to be y=-1/2x+3/2 and then substitute x=4 and y=y in the above equation. Got -7/2 as the answer. Can anyone please shed what's wrong with my approach?

I think the line AB intersects the y-axis at (0,3) => y = (-1/2)x + 3
Intern  Joined: 09 Nov 2016
Posts: 5
Location: India
Concentration: Marketing, General Management
GPA: 3.6
Re: Beauty of Coordinate geometry  [#permalink]

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I just made a very silly mistake. Thankyou chesstitans
Non-Human User Joined: 09 Sep 2013
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Re: Beauty of Coordinate geometry  [#permalink]

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Hello from the GMAT Club BumpBot!

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_________________ Re: Beauty of Coordinate geometry   [#permalink] 27 Dec 2018, 17:55
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