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I don't understand this step "Subtract (2) and (4)"
Can you elaborate pls?
Thanks
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\(AP^2 = (4+x)^2+(\frac{3}{2})^2\).......(2)

\((10-AP)^2=(x-4)^2+(\frac{3}{2})^2\) ..........(4)

From equation (2) and (4)

\(AP^2- (4+x^2) = (10-AP)^2 -(x-4)^2\)

\(AP^2 -16-x^2-8x = 100+AP^2 -20*AP -x^2-16+8x\)

\(20*AP = 16x+100\)

\(AP = \frac{4x}{5} + 5\)





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nick1816
I don't understand this step "Subtract (2) and (4)"
Can you elaborate pls?
Thanks
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Points C and P lie on the circumference of an ellipse.

The equation of the ellipse is given by \(\frac{x^2}{5^2} + \frac{y^2}{3^2} = 1\)
Substitute \(y=\frac{3}{2}\) in the equation. This will yield \(x^2 = \frac{75}{4}\)

Now, \(CP^2 = x^2 + y^2 = \frac{75}{4} + \frac{9}{4} = 21\)

=> \(CP = \sqrt{21}\)

nick1816, do we need to know the equation of an ellipse to answer this? Is there any other approach?

Thanks
Lipun
may i ask how did you understand it is on an elipse?
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ellipse is the locus of a point which moves in a plane such that the sum of its distances from the two points called foci add up to a constant.

AP+BP+ AB = AC+BC+AB

or AP+BP = AC+BC

The sum of distance of P from A and B is same as that of C from A and B. (similar to the definition of a ellipse)

AnirudhaS
Lipun
Points C and P lie on the circumference of an ellipse.

The equation of the ellipse is given by \(\frac{x^2}{5^2} + \frac{y^2}{3^2} = 1\)
Substitute \(y=\frac{3}{2}\) in the equation. This will yield \(x^2 = \frac{75}{4}\)

Now, \(CP^2 = x^2 + y^2 = \frac{75}{4} + \frac{9}{4} = 21\)

=> \(CP = \sqrt{21}\)

nick1816, do we need to know the equation of an ellipse to answer this? Is there any other approach?

Thanks
Lipun
may i ask how did you understand it is on an elipse?
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Lipun
1 number bhai! But we don't need to know ellipse to answer this question.

Suppose coordinates of P =(x,y)
Since the △ABP has half the area of △ABC and both triangle have common base, y must be equal to 3/2.

Also, AC=BC=5, hence AP+PB = 5+5=10........(1) [perimeter of both triangles are equal]

\(AP^2 = (4+x)^2+(\frac{3}{2})^2\).......(2)

\(PB^2= (x-4)^2+(\frac{3}{2})^2\)......(3)

\((10-AP)^2=(x-4)^2+(\frac{3}{2})^2\) ..........(4)

Subtract (2) and (4)

\(AP = 5+ \frac{4x}{5}\)

Put value of AP in equation 2
\(x^2 = \frac{75}{4}\)

\(CP^2 = \frac{75}{4}+\frac{9}{4} = 21\)

\(CP = \sqrt{21}\)


Hi nick1816,

I didn't understand your method. First I'll tell you how and till where I was able to solve this.

Base of △ABC is 8 units and Height is 3 units
Area of △ABC = 12.

Area of △ABP = 6. GIVEN.
So Height for ABP is 3/2 since ABC and ABP share the same Base.

So here are my following doubts.

1. How were you able to identify that the Y coordinate of Point P is its height or did you just assume.? Or is there a property which says that the X coordinate has to be the Base and the Y coordinate has to be the Height?

2. I got it till the AC=BC=5 and AP+PB=10 part. But I was totally lost after that. You used the pythagoras' theorem but with respect which triangles and which sides?

Because AP and PB doesn't look like hypotenuse.

Please help me understand this. It'll be great.

I'm not an engineer plus not so good at math so pardon my lack of ability to understand YOUR way.

Tagging others just in case nick1816 is offline.
yashikaaggarwal chetan2u Bunuel IanStewart GMATinsight ScottTargetTestPrep fskilnik

Thank you. :)

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nick1816
\(\triangle ABC\) has coordinates A= (-4, 0), B= (4 , 0), and C= (0 , 3). Let P be the point in the first quadrant such that \(\triangle ABP\) has half the area of \(\triangle ABC\) but both triangles have the same perimeter. What is the length of CP?
Attachment:
hyrCFIlaO6-92280.png

A. \(\sqrt{15}\)
B. 4
C. \(\sqrt{17}\)
D. \(\sqrt{21}\)
E. 5

Answer: Option D

Please check the video for the step-by-step solution.

GMATinsight's Solution




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Nups1324

1. How were you able to identify that the Y coordinate of Point P is its height or did you just assume.? Or is there a property which says that the X coordinate has to be the Base and the Y coordinate has to be the Height?

I wouldn't even look at this question unless you're near the top end of the Quant scoring scale. You should really be practicing questions that are at or just slightly above your level, and this is a very high-level problem.

When you choose a base for a triangle, you find the triangle's height by:

- extending the base so it is infinitely long (if you need to)
- measuring the perpendicular distance from that infinitely long base to the third point of the triangle

So for triangle APB here, if we decide AB will be our base, we first imagine it is an infinitely long line, so in this diagram, our base becomes the x-axis. Then to find the triangle's height, we want to find the perpendicular distance from point P to the x-axis. That distance will just be the y-coordinate of point P, since we're measuring the distance in a perfectly vertical direction.

Once you know point P is at (x, 3/2), you can then use the coordinate geometry distance formula (twice) to find the perimeter of APB. That distance formula is just based on the Pythagorean Theorem, which is why nick's solution above appears to use Pythagoras. But if you're unfamiliar with how to find distances in coordinate geometry, this is definitely not the first question you should practice - you should do much easier questions than this one first.
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Nups1324

1. How were you able to identify that the Y coordinate of Point P is its height or did you just assume.? Or is there a property which says that the X coordinate has to be the Base and the Y coordinate has to be the Height?

I wouldn't even look at this question unless you're near the top end of the Quant scoring scale. You should really be practicing questions that are at or just slightly above your level, and this is a very high-level problem.

When you choose a base for a triangle, you find the triangle's height by:

- extending the base so it is infinitely long (if you need to)
- measuring the perpendicular distance from that infinitely long base to the third point of the triangle

So for triangle APB here, if we decide AB will be our base, we first imagine it is an infinitely long line, so in this diagram, our base becomes the x-axis. Then to find the triangle's height, we want to find the perpendicular distance from point P to the x-axis. That distance will just be the y-coordinate of point P, since we're measuring the distance in a perfectly vertical direction.

Once you know point P is at (x, 3/2), you can then use the coordinate geometry distance formula (twice) to find the perimeter of APB. That distance formula is just based on the Pythagorean Theorem, which is why nick's solution above appears to use Pythagoras. But if you're unfamiliar with how to find distances in coordinate geometry, this is definitely not the first question you should practice - you should do much easier questions than this one first.


Hi IanStewart,

I think you're absolutely correct. I should keep on revising my basics and focus on the basic questions first. I'll use this method in my overall strategy. I hope that one day I'll be able to solve at the highest level and I will.

Thank you :)

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