# Circle and a line dividing the circumference

• Navin
In summary, the conversation discusses finding the equation of a circle that is divided by a given line in a 1:2 ratio. The attempt at a solution involves finding the distance of the center of the circle from the chord, making a relation between the radius and the common factor of the ratio, and using circle functions and trigonometry to calculate the radius and ultimately, the equation of the circle. However, there are discrepancies in the calculations and the final equation given may not be correct.f

## Homework Statement

The line "4x + 3y -4=0 " divides the circumference of circle centred at (5,3) in ratio 1:2.whats the equation of the circle

## Homework Equations

2πr
Distance of a line from point

## The Attempt at a Solution

I tried finding the distance of the centre from the chord.
Then i made a relation between radius and common factor of the ratio between the two circumfernce divisions
And now I am stuck :(

you can check out “circle functions” in detail on the internet.

you can check out “circle functions” in detail on the internet.
How do circle functions help here ?
They haven't given any angle or what's so ever .

(Just an indication)
If you assume the central angle of the triangle made by the two points which the line and the circle and the center as alpha , the area of it will be:

Then you can use the conditions mentioned to calculate the area of this triangle,then calculate
the radius by the equation above.

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Thank you so much !

YoungPhysicist
Thank you so much !
What is your result & how did you get it?

(We always like to know after helping someone.)

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Okay sorry for the delay SammyS.
So here's what i did

i used the circle function to relate the two circum ference. ( arc length = Radius × Angle in radians) and i got the value for alpha whivh was the angle i asumed.

Then i used angle sum property of a triangle and found out that twas a equilateral triangle of angle 60°(i don't think there are other types of equilaterals in a cartisian plane !)

Then i found the perpendicular distance between the centre of circle and the line which came out to be one (using distance formula between line and point)

And then i related radius and that distance by using sin 60° and got radius.

Then by using the radius formula of general circle i found C.

And hence the equation came to be
x2 +y2 -10x -6y +30 = 0

I pray i didnt do a calculation error

No that's not right, and you didn't give a lot of detail on the individual steps.
i used the circle function to relate the two circumference. ( arc length = Radius × Angle in radians) and i got the value for alpha which was the angle i assumed.
I could ask what you got for the angle, but probably more important: What was the arc length you used to get the angle, and how does that arc length compare to the circumference of the circle?
Then i used angle sum property of a triangle and found out that it was a equilateral triangle of angle 60°(i don't think there are other types of equilaterals in a Cartesian plane !)
It's not clear how this triangle is oriented with respect to the circle or to the line.
Then i found the perpendicular distance between the centre of circle and the line which came out to be one (using distance formula between line and point)
What is the equation of the line which is perpendicular to the given line?

That is not the correct distance.
And then i related radius and that distance by using sin 60° and got radius.
Then by using the radius formula of general circle i found C.
What is this C?

Give this formula.
And hence the equation came to be
x2 +y2 -10x -6y +30 = 0

I pray i didnt do a calculation error
That is the equation of a circle centered at (5, 3), and its radius is 2.

Is that what you intended?

The graph given by @Young physicist in post (#4) is not necessarily to scale, but ifs clear that when you consider the x and y intercepts of the given line, the radius of the circle needs to be much greater than 2 to have any points in common with the given line.

YoungPhysicist
The graph given by @Young physicist in post (#4) is not necessarily to scale,
Just giving a general concept

Just giving a general concept

This diagram is pretty much on the mark. I would print it out and then draw the line from centre of circle to (calculated) midpoint of the chord. Mark the angles you can obtain from the circumference ratio and the rest should be simple trig!

YoungPhysicist
No that's not right, and you didn't give a lot of detail on the individual steps.
I could ask what you got for the angle, but probably more important: What was the arc length you used to get the angle, and how does that arc length compare to the circumference of the circle?
It's not clear how this triangle is oriented with respect to the circle or to the line.
What is the equation of the line which is perpendicular to the given line?

That is not the correct distance.
What is this C?

Give this formula.

That is the equation of a circle centered at (5, 3), and its radius is 2.

Is that what you intended?

The graph given by @Young physicist in post (#4) is not necessarily to scale, but ifs clear that when you consider the x and y intercepts of the given line, the radius of the circle needs to be much greater than 2 to have any points in common with the given line.
Yes i just realized that my distance is wrong.it should be 5.(i forgot to root something)

Wait ill just post a detailed solutiin in a few min

YoungPhysicist
Ps - is it compulsary that i type the full soln or can i just send an attachement with the handwritten working ? Someone had said it was against forum policy to do the later.just confirming

Okay so here's what i did.

So first i calculated Arc Length
Now the ratio of circumference is 1:2.
Let the common factor be x
S = r θ where θ is central angle of the minor arc
x =r ×θ...
X/θ = r (1)

Lets look at major arc

2X = r ×(2π-θ)
2X= (X/θ)(2π-θ)

2θ=2π-θ
3θ=2π
θ =2π/3

Converting to degrees we get (2 ×180)/3=120°
(Not 60 as i previously screwed up in calculations)

YoungPhysicist
Now let's look at the distance tween cente and the line

This is given by | (ax + by + k)÷(root of (a2 +b2|

Where a b and k are constants in the equation of line and x and y are coardinates of the point
And" | | "represent modulus

With that we get
| (4 ×5 + 3×3 -4) ÷ 5| = 5(not 1 as i previosly did)

YoungPhysicist
If we look at that triangle and use angle sum prop
We get Φ +Φ +120 = 180

Φ=30

Hence now we can finaly find radius
By using sin 30 =opposite side÷hypotenuese

(1/2) =(5/r)
r =10

With this we can use the formula for radius

g2 + f2 +c = 10
Where -g and -h are coord nates of centre

25 +9 +c =10

c= -24

And now that we have all unknows the equation of general circle

x2 +y2 + 2gx +2fy +c =0

Hold on i made an error

The formula for radius is g2 + f2 -c = 10

Hence r = -24

Anyway so equation of circle is

x2 +y2 -10x -6y -24 =0

YoungPhysicist
I checked the answer given and its not matching

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YoungPhysicist
We r suposed to get
x^2 +y^2 -10x -6y -66 =0

We r suposed to get
x^2 +y^2 -10x -6y -66 =0
Any ideas ?

So first i calculated Arc Length
Now the ratio of circumference is 1:2.
Let the common factor be x
S = r θ where θ is central angle of the minor arc
x =r ×θ...
X/θ = r (1)

Lets look at major arc

2X = r ×(2π-θ)
2X= (X/θ)(2π-θ)

2θ=2π-θ
3θ=2π
θ =2π/3

Converting to degrees we get (2 ×180)/3=120°
(Not 60 as i previously screwed up in calculations)
That's good !

Also, the distance from (5,3) to the line 4x + 3y − 4 = 0 is indeed 5, as you found.

Navin
That's good !

Also, the distance from (5,3) to the line 4x + 3y − 4 = 0 is indeed 5, as you found.
But the equation I am getting is wrong ! Where did i make an error ?

If we look at that triangle and use angle sum prop
We get Φ +Φ +120 = 180

Φ=30
Hence now we can finaly find radius
By using sin 30 =opposite side÷hypotenuese

(1/2) =(5/r)
r =10
That looks O.K., unless I'm missing something.

Also ##\ \displaystyle \cos\left(\frac{\theta}{2}\right) = \frac{5}{R} \ ## should work, for θ = 120°.

And now that we have all unknows the equation of general circle

x2 +y2 + 2gx +2fy +c =0

Hold on i made an error

The formula for radius is g2 + f2 -c = 10

Hence r = -24
I would use the following for the formula of a circle, radius R and centered at point ( h, k ) :

## \displaystyle (x-h)^2 + (y-k)^2 = R^2 ##​
.
To get the equation in the form you want, expand the those squared terms and simplify.

With this we can use the formula for radius

g2 + f2 +c = 10
Where -g and -h are coord nates of centre

25 +9 +c =10

c= -24
After looking at this again, shouldn't this be radius squared?
:
## (-g)^2 + (-h)^2+c = R^2 \,,\ ## where g and h are coordinates of the centre and R is the radius. In your case, R = 10 .

Navin
After looking at this again, shouldn't this be radius squared?
:
## (-g)^2 + (-h)^2+c = R^2 \,,\ ## where g and h are coordinates of the centre and R is the radius. In your case, R = 10 .
*face palm*

So
*face palm*
It will be

100 -24-9
=100-33
=66
...yep so the value of c is 66
Hence the circle will be

x^2 +y^2 -10x -6y -66 =0

Yay ! FINALY !

SammyS
Thank you Sammy and A Young Physist And All those who helped me

You all get a virtual cookie
...mmmmm taste those bytes(get the pun)

YoungPhysicist
You all get a virtual cookie
...mmmmm taste those bytes(get the pun)
Let's see... How many bits are in each byte?

... and are these bits similar to crumbs - in the case of a virtual cookie ?

YoungPhysicist