Expected area of triangle inscribed in a circle
$begingroup$
On a unit circle , BC is a chord with length 4/π . Point A is picked randomly at its circumference .
Find the expected area of △ ABC .
geometric-probability
$endgroup$
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$begingroup$
On a unit circle , BC is a chord with length 4/π . Point A is picked randomly at its circumference .
Find the expected area of △ ABC .
geometric-probability
$endgroup$
add a comment |
$begingroup$
On a unit circle , BC is a chord with length 4/π . Point A is picked randomly at its circumference .
Find the expected area of △ ABC .
geometric-probability
$endgroup$
On a unit circle , BC is a chord with length 4/π . Point A is picked randomly at its circumference .
Find the expected area of △ ABC .
geometric-probability
geometric-probability
asked Jan 10 at 6:56
mrwongmrwong
5915
5915
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1 Answer
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$begingroup$
For easy reference let chord BC sits in the lower portion of the unit circle. It lies horizontally making angle $2phi$ at the centre of the circle symmetric to the y axis.
Then the perpendicular distance of the chord from the centre is $D = sqrt{(1 - (frac{2}{pi})^2}$ and $sinphi = frac{2}{pi}$
Let the line joining A and the centre makes an angle $theta$ with the x axis, then
For $theta$ between $0$ and $frac{pi}{2}$,
$$Area = frac{1}{2}frac{4}{pi}(sintheta + D)$$
For $theta$ between $0$ and $frac{pi}{2} - phi$
$$Area = frac{1}{2}frac{4}{pi}(D - sintheta)$$
For $theta$ between $frac{pi}{2} - phi$ and $frac{pi}{2}$
$$Area = frac{1}{2}frac{4}{pi}(sintheta - D)$$
Hence the average area is
$$frac{4}{2pi^2}(int_0^{frac{pi}{2}}(sintheta + D)dtheta + int_0^{frac{pi}{2} - phi}(D - sintheta)dtheta + int_{frac{pi}{2} - phi}^{frac{pi}{2}}(sintheta - D)dtheta)$$
$$= frac{2}{pi^2}(frac{2}{pi} + Dpi - 2Dphi + sinphi) = 0.533$$
I managed to borrow a PC and use 1000 random points and the result is around 5,1 which is very close to the calculation.
Also if the area is $frac{3}{2pi}$, I have used trial and error to find x to be about 1.54 which is your expected answer.
$endgroup$
$begingroup$
Thanks for your reply .You got a result of about 0.7 sq.unit . Will you please check the result with simulation . If they match then I will accept the result .
$endgroup$
– mrwong
Jan 12 at 4:04
$begingroup$
We can estimate the area for those triangles above the chord. As D is 0.77 and the chord length is 1.27. The simple average height of these triangles is (1+0.77)/2 = 0.885. Therefore the area is 1/2. x 0.885 x 1.27 = 0.562. Similarly the simple area average is 1/2 x 0.115 x 1.27 = 0.07 for those triangles below the chord. Since the % of height over 0.885 is higher, the answer of 0.7 is reasonable.
$endgroup$
– KY Tang
Jan 13 at 1:37
$begingroup$
:Thanks again . If the question is changed to : The length of chord BC = x units , while A is picked randomly . If the expected area of △ ABC = 3/ 2π (about 0.477 sq.un) , then what is the value of x ?
$endgroup$
– mrwong
Jan 13 at 5:08
$begingroup$
In fact the formula in terms of D and phi is 1/(pi^2) x (4sinphi + 2piD). Now D = sqrt(1-x^2/4) and sin phi = x/2. Then put this into the area formula we have x = 1.99. That means the chord is almost the diameter. Please see if this is what you expect.
$endgroup$
– KY Tang
Jan 13 at 17:16
$begingroup$
I got an answer of about 1.53 elsewhere . Is it useful to use simulation to clarify ?
$endgroup$
– mrwong
Jan 14 at 4:40
|
show 7 more comments
Your Answer
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1 Answer
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1 Answer
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$begingroup$
For easy reference let chord BC sits in the lower portion of the unit circle. It lies horizontally making angle $2phi$ at the centre of the circle symmetric to the y axis.
Then the perpendicular distance of the chord from the centre is $D = sqrt{(1 - (frac{2}{pi})^2}$ and $sinphi = frac{2}{pi}$
Let the line joining A and the centre makes an angle $theta$ with the x axis, then
For $theta$ between $0$ and $frac{pi}{2}$,
$$Area = frac{1}{2}frac{4}{pi}(sintheta + D)$$
For $theta$ between $0$ and $frac{pi}{2} - phi$
$$Area = frac{1}{2}frac{4}{pi}(D - sintheta)$$
For $theta$ between $frac{pi}{2} - phi$ and $frac{pi}{2}$
$$Area = frac{1}{2}frac{4}{pi}(sintheta - D)$$
Hence the average area is
$$frac{4}{2pi^2}(int_0^{frac{pi}{2}}(sintheta + D)dtheta + int_0^{frac{pi}{2} - phi}(D - sintheta)dtheta + int_{frac{pi}{2} - phi}^{frac{pi}{2}}(sintheta - D)dtheta)$$
$$= frac{2}{pi^2}(frac{2}{pi} + Dpi - 2Dphi + sinphi) = 0.533$$
I managed to borrow a PC and use 1000 random points and the result is around 5,1 which is very close to the calculation.
Also if the area is $frac{3}{2pi}$, I have used trial and error to find x to be about 1.54 which is your expected answer.
$endgroup$
$begingroup$
Thanks for your reply .You got a result of about 0.7 sq.unit . Will you please check the result with simulation . If they match then I will accept the result .
$endgroup$
– mrwong
Jan 12 at 4:04
$begingroup$
We can estimate the area for those triangles above the chord. As D is 0.77 and the chord length is 1.27. The simple average height of these triangles is (1+0.77)/2 = 0.885. Therefore the area is 1/2. x 0.885 x 1.27 = 0.562. Similarly the simple area average is 1/2 x 0.115 x 1.27 = 0.07 for those triangles below the chord. Since the % of height over 0.885 is higher, the answer of 0.7 is reasonable.
$endgroup$
– KY Tang
Jan 13 at 1:37
$begingroup$
:Thanks again . If the question is changed to : The length of chord BC = x units , while A is picked randomly . If the expected area of △ ABC = 3/ 2π (about 0.477 sq.un) , then what is the value of x ?
$endgroup$
– mrwong
Jan 13 at 5:08
$begingroup$
In fact the formula in terms of D and phi is 1/(pi^2) x (4sinphi + 2piD). Now D = sqrt(1-x^2/4) and sin phi = x/2. Then put this into the area formula we have x = 1.99. That means the chord is almost the diameter. Please see if this is what you expect.
$endgroup$
– KY Tang
Jan 13 at 17:16
$begingroup$
I got an answer of about 1.53 elsewhere . Is it useful to use simulation to clarify ?
$endgroup$
– mrwong
Jan 14 at 4:40
|
show 7 more comments
$begingroup$
For easy reference let chord BC sits in the lower portion of the unit circle. It lies horizontally making angle $2phi$ at the centre of the circle symmetric to the y axis.
Then the perpendicular distance of the chord from the centre is $D = sqrt{(1 - (frac{2}{pi})^2}$ and $sinphi = frac{2}{pi}$
Let the line joining A and the centre makes an angle $theta$ with the x axis, then
For $theta$ between $0$ and $frac{pi}{2}$,
$$Area = frac{1}{2}frac{4}{pi}(sintheta + D)$$
For $theta$ between $0$ and $frac{pi}{2} - phi$
$$Area = frac{1}{2}frac{4}{pi}(D - sintheta)$$
For $theta$ between $frac{pi}{2} - phi$ and $frac{pi}{2}$
$$Area = frac{1}{2}frac{4}{pi}(sintheta - D)$$
Hence the average area is
$$frac{4}{2pi^2}(int_0^{frac{pi}{2}}(sintheta + D)dtheta + int_0^{frac{pi}{2} - phi}(D - sintheta)dtheta + int_{frac{pi}{2} - phi}^{frac{pi}{2}}(sintheta - D)dtheta)$$
$$= frac{2}{pi^2}(frac{2}{pi} + Dpi - 2Dphi + sinphi) = 0.533$$
I managed to borrow a PC and use 1000 random points and the result is around 5,1 which is very close to the calculation.
Also if the area is $frac{3}{2pi}$, I have used trial and error to find x to be about 1.54 which is your expected answer.
$endgroup$
$begingroup$
Thanks for your reply .You got a result of about 0.7 sq.unit . Will you please check the result with simulation . If they match then I will accept the result .
$endgroup$
– mrwong
Jan 12 at 4:04
$begingroup$
We can estimate the area for those triangles above the chord. As D is 0.77 and the chord length is 1.27. The simple average height of these triangles is (1+0.77)/2 = 0.885. Therefore the area is 1/2. x 0.885 x 1.27 = 0.562. Similarly the simple area average is 1/2 x 0.115 x 1.27 = 0.07 for those triangles below the chord. Since the % of height over 0.885 is higher, the answer of 0.7 is reasonable.
$endgroup$
– KY Tang
Jan 13 at 1:37
$begingroup$
:Thanks again . If the question is changed to : The length of chord BC = x units , while A is picked randomly . If the expected area of △ ABC = 3/ 2π (about 0.477 sq.un) , then what is the value of x ?
$endgroup$
– mrwong
Jan 13 at 5:08
$begingroup$
In fact the formula in terms of D and phi is 1/(pi^2) x (4sinphi + 2piD). Now D = sqrt(1-x^2/4) and sin phi = x/2. Then put this into the area formula we have x = 1.99. That means the chord is almost the diameter. Please see if this is what you expect.
$endgroup$
– KY Tang
Jan 13 at 17:16
$begingroup$
I got an answer of about 1.53 elsewhere . Is it useful to use simulation to clarify ?
$endgroup$
– mrwong
Jan 14 at 4:40
|
show 7 more comments
$begingroup$
For easy reference let chord BC sits in the lower portion of the unit circle. It lies horizontally making angle $2phi$ at the centre of the circle symmetric to the y axis.
Then the perpendicular distance of the chord from the centre is $D = sqrt{(1 - (frac{2}{pi})^2}$ and $sinphi = frac{2}{pi}$
Let the line joining A and the centre makes an angle $theta$ with the x axis, then
For $theta$ between $0$ and $frac{pi}{2}$,
$$Area = frac{1}{2}frac{4}{pi}(sintheta + D)$$
For $theta$ between $0$ and $frac{pi}{2} - phi$
$$Area = frac{1}{2}frac{4}{pi}(D - sintheta)$$
For $theta$ between $frac{pi}{2} - phi$ and $frac{pi}{2}$
$$Area = frac{1}{2}frac{4}{pi}(sintheta - D)$$
Hence the average area is
$$frac{4}{2pi^2}(int_0^{frac{pi}{2}}(sintheta + D)dtheta + int_0^{frac{pi}{2} - phi}(D - sintheta)dtheta + int_{frac{pi}{2} - phi}^{frac{pi}{2}}(sintheta - D)dtheta)$$
$$= frac{2}{pi^2}(frac{2}{pi} + Dpi - 2Dphi + sinphi) = 0.533$$
I managed to borrow a PC and use 1000 random points and the result is around 5,1 which is very close to the calculation.
Also if the area is $frac{3}{2pi}$, I have used trial and error to find x to be about 1.54 which is your expected answer.
$endgroup$
For easy reference let chord BC sits in the lower portion of the unit circle. It lies horizontally making angle $2phi$ at the centre of the circle symmetric to the y axis.
Then the perpendicular distance of the chord from the centre is $D = sqrt{(1 - (frac{2}{pi})^2}$ and $sinphi = frac{2}{pi}$
Let the line joining A and the centre makes an angle $theta$ with the x axis, then
For $theta$ between $0$ and $frac{pi}{2}$,
$$Area = frac{1}{2}frac{4}{pi}(sintheta + D)$$
For $theta$ between $0$ and $frac{pi}{2} - phi$
$$Area = frac{1}{2}frac{4}{pi}(D - sintheta)$$
For $theta$ between $frac{pi}{2} - phi$ and $frac{pi}{2}$
$$Area = frac{1}{2}frac{4}{pi}(sintheta - D)$$
Hence the average area is
$$frac{4}{2pi^2}(int_0^{frac{pi}{2}}(sintheta + D)dtheta + int_0^{frac{pi}{2} - phi}(D - sintheta)dtheta + int_{frac{pi}{2} - phi}^{frac{pi}{2}}(sintheta - D)dtheta)$$
$$= frac{2}{pi^2}(frac{2}{pi} + Dpi - 2Dphi + sinphi) = 0.533$$
I managed to borrow a PC and use 1000 random points and the result is around 5,1 which is very close to the calculation.
Also if the area is $frac{3}{2pi}$, I have used trial and error to find x to be about 1.54 which is your expected answer.
edited Jan 15 at 1:27
answered Jan 11 at 1:50
KY TangKY Tang
1245
1245
$begingroup$
Thanks for your reply .You got a result of about 0.7 sq.unit . Will you please check the result with simulation . If they match then I will accept the result .
$endgroup$
– mrwong
Jan 12 at 4:04
$begingroup$
We can estimate the area for those triangles above the chord. As D is 0.77 and the chord length is 1.27. The simple average height of these triangles is (1+0.77)/2 = 0.885. Therefore the area is 1/2. x 0.885 x 1.27 = 0.562. Similarly the simple area average is 1/2 x 0.115 x 1.27 = 0.07 for those triangles below the chord. Since the % of height over 0.885 is higher, the answer of 0.7 is reasonable.
$endgroup$
– KY Tang
Jan 13 at 1:37
$begingroup$
:Thanks again . If the question is changed to : The length of chord BC = x units , while A is picked randomly . If the expected area of △ ABC = 3/ 2π (about 0.477 sq.un) , then what is the value of x ?
$endgroup$
– mrwong
Jan 13 at 5:08
$begingroup$
In fact the formula in terms of D and phi is 1/(pi^2) x (4sinphi + 2piD). Now D = sqrt(1-x^2/4) and sin phi = x/2. Then put this into the area formula we have x = 1.99. That means the chord is almost the diameter. Please see if this is what you expect.
$endgroup$
– KY Tang
Jan 13 at 17:16
$begingroup$
I got an answer of about 1.53 elsewhere . Is it useful to use simulation to clarify ?
$endgroup$
– mrwong
Jan 14 at 4:40
|
show 7 more comments
$begingroup$
Thanks for your reply .You got a result of about 0.7 sq.unit . Will you please check the result with simulation . If they match then I will accept the result .
$endgroup$
– mrwong
Jan 12 at 4:04
$begingroup$
We can estimate the area for those triangles above the chord. As D is 0.77 and the chord length is 1.27. The simple average height of these triangles is (1+0.77)/2 = 0.885. Therefore the area is 1/2. x 0.885 x 1.27 = 0.562. Similarly the simple area average is 1/2 x 0.115 x 1.27 = 0.07 for those triangles below the chord. Since the % of height over 0.885 is higher, the answer of 0.7 is reasonable.
$endgroup$
– KY Tang
Jan 13 at 1:37
$begingroup$
:Thanks again . If the question is changed to : The length of chord BC = x units , while A is picked randomly . If the expected area of △ ABC = 3/ 2π (about 0.477 sq.un) , then what is the value of x ?
$endgroup$
– mrwong
Jan 13 at 5:08
$begingroup$
In fact the formula in terms of D and phi is 1/(pi^2) x (4sinphi + 2piD). Now D = sqrt(1-x^2/4) and sin phi = x/2. Then put this into the area formula we have x = 1.99. That means the chord is almost the diameter. Please see if this is what you expect.
$endgroup$
– KY Tang
Jan 13 at 17:16
$begingroup$
I got an answer of about 1.53 elsewhere . Is it useful to use simulation to clarify ?
$endgroup$
– mrwong
Jan 14 at 4:40
$begingroup$
Thanks for your reply .You got a result of about 0.7 sq.unit . Will you please check the result with simulation . If they match then I will accept the result .
$endgroup$
– mrwong
Jan 12 at 4:04
$begingroup$
Thanks for your reply .You got a result of about 0.7 sq.unit . Will you please check the result with simulation . If they match then I will accept the result .
$endgroup$
– mrwong
Jan 12 at 4:04
$begingroup$
We can estimate the area for those triangles above the chord. As D is 0.77 and the chord length is 1.27. The simple average height of these triangles is (1+0.77)/2 = 0.885. Therefore the area is 1/2. x 0.885 x 1.27 = 0.562. Similarly the simple area average is 1/2 x 0.115 x 1.27 = 0.07 for those triangles below the chord. Since the % of height over 0.885 is higher, the answer of 0.7 is reasonable.
$endgroup$
– KY Tang
Jan 13 at 1:37
$begingroup$
We can estimate the area for those triangles above the chord. As D is 0.77 and the chord length is 1.27. The simple average height of these triangles is (1+0.77)/2 = 0.885. Therefore the area is 1/2. x 0.885 x 1.27 = 0.562. Similarly the simple area average is 1/2 x 0.115 x 1.27 = 0.07 for those triangles below the chord. Since the % of height over 0.885 is higher, the answer of 0.7 is reasonable.
$endgroup$
– KY Tang
Jan 13 at 1:37
$begingroup$
:Thanks again . If the question is changed to : The length of chord BC = x units , while A is picked randomly . If the expected area of △ ABC = 3/ 2π (about 0.477 sq.un) , then what is the value of x ?
$endgroup$
– mrwong
Jan 13 at 5:08
$begingroup$
:Thanks again . If the question is changed to : The length of chord BC = x units , while A is picked randomly . If the expected area of △ ABC = 3/ 2π (about 0.477 sq.un) , then what is the value of x ?
$endgroup$
– mrwong
Jan 13 at 5:08
$begingroup$
In fact the formula in terms of D and phi is 1/(pi^2) x (4sinphi + 2piD). Now D = sqrt(1-x^2/4) and sin phi = x/2. Then put this into the area formula we have x = 1.99. That means the chord is almost the diameter. Please see if this is what you expect.
$endgroup$
– KY Tang
Jan 13 at 17:16
$begingroup$
In fact the formula in terms of D and phi is 1/(pi^2) x (4sinphi + 2piD). Now D = sqrt(1-x^2/4) and sin phi = x/2. Then put this into the area formula we have x = 1.99. That means the chord is almost the diameter. Please see if this is what you expect.
$endgroup$
– KY Tang
Jan 13 at 17:16
$begingroup$
I got an answer of about 1.53 elsewhere . Is it useful to use simulation to clarify ?
$endgroup$
– mrwong
Jan 14 at 4:40
$begingroup$
I got an answer of about 1.53 elsewhere . Is it useful to use simulation to clarify ?
$endgroup$
– mrwong
Jan 14 at 4:40
|
show 7 more comments
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