# Circonferenza per tre punti: centro, raggio, equazione

> Trova centro, raggio, area ed equazione della circonferenza per tre punti, nella forma (x − h)² + (y − k)² = r² e nella forma generale.

Versione interattiva: https://www.calcopenly.com/it/geometry/circle-through-three-points
Argomento: Calcolatori di geometria

Three points that do not lie on one line fix exactly one circle, the circumcircle of the triangle they form. Its centre (h, k) is where the perpendicular bisectors of two chords meet, which a determinant formula gives directly, and the radius is the distance from the centre to any of the points. Coordinates stay exact fractions, so a centre at (1, 4/3) is shown as 4/3, not 1.3333.

The default points (−3, 4), (4, 5) and (1, −4) give centre (1, 1) and radius 5: (x − 1)² + (y − 1)² = 25, or x² + y² − 2x − 2y − 23 = 0. The same construction finds the centre of a round table, pipe or arch from three marks on its edge, and the radius of a road curve from three survey points.

If the determinant is zero the points are collinear and no circle exists. Coordinates carry no unit; the radius and area are in the coordinates' unit and its square.

## Dati

- **Point 1: x**
- **Point 1: y**
- **Point 2: x**
- **Point 2: y**
- **Point 3: x**
- **Point 3: y**

## Risultati

- Raggio — risultato principale
- Centre x
- Centre y
- Standard form
- General form
- Diametro
- Circonferenza
- Area

## Formula

$$
\begin{gathered} h = \frac{\sum (x_i^2 + y_i^2)(y_j - y_k)}{2\sum x_i(y_j - y_k)},\quad k = \frac{\sum (x_i^2 + y_i^2)(x_k - x_j)}{2\sum x_i(y_j - y_k)} \\[6pt] (x - h)^2 + (y - k)^2 = r^2 \end{gathered}
$$

## Esempi svolti

### (−3, 4), (4, 5), (1, −4)

- Point 1: x: -3
- Point 1: y: 4
- Point 2: x: 4
- Point 2: y: 5
- Point 3: x: 1
- Point 3: y: -4
- **Centre x: 1**
- **Centre y: 1**
- **Raggio: 5**
- **Standard form: (x − 1)² + (y − 1)² = 25**
- **General form: x² + y² − 2x − 2y − 23 = 0**
- Fonte di verifica: Python 3.8 fractions with the determinant formula; each point is 5 from (1, 1): 4² + 3², 3² + 4², 0² + 5²

### Right-angled corner (0, 0), (4, 0), (0, 3)

- Point 1: x: 0
- Point 1: y: 0
- Point 2: x: 4
- Point 2: y: 0
- Point 3: x: 0
- Point 3: y: 3
- **Centre x: 2**
- **Centre y: 1.5**
- **Raggio: 2.5**
- **Area: 19.634954**
- Fonte di verifica: Thales: the hypotenuse (length 5) is a diameter, centre at its midpoint; Python 3.8 math: pi*2.5**2

### Centre at the origin (edge case: no shift terms)

- Point 1: x: 1
- Point 1: y: 0
- Point 2: x: 0
- Point 2: y: 1
- Point 3: x: -1
- Point 3: y: 0
- **Centre x: 0**
- **Centre y: 0**
- **Raggio: 1**
- **Standard form: x² + y² = 1**
- **General form: x² + y² − 1 = 0**
- Fonte di verifica: All three points are 1 from the origin

### Fractional centre (0, 0), (2, 0), (1, 3)

- Point 1: x: 0
- Point 1: y: 0
- Point 2: x: 2
- Point 2: y: 0
- Point 3: x: 1
- Point 3: y: 3
- **Centre x: 1**
- **Centre y: 1.33333333**
- **Raggio: 1.66666667**
- **Standard form: (x − 1)² + (y − 4/3)² = 25/9**
- Fonte di verifica: Python 3.8 fractions: h = 1 by symmetry, 9 − 6k = 1 gives k = 4/3, r² = 1 + 16/9 = 25/9

## Domande

### How do you find the equation of a circle through three points?

Substitute each point into the general form x² + y² + Dx + Ey + F = 0 and solve the three linear equations for D, E and F. For (−3, 4), (4, 5) and (1, −4) this gives D = −2, E = −2 and F = −23. Completing the square turns that into (x − 1)² + (y − 1)² = 25: centre (1, 1), radius 5.

### How do you find the centre of a circle from three points on it?

Construct the perpendicular bisectors of two chords, such as P₁P₂ and P₂P₃; they cross at the centre, because every point on a perpendicular bisector is equally far from both ends of its chord. For a right triangle the centre is the midpoint of the hypotenuse: (0, 0), (4, 0) and (0, 3) give centre (2, 1.5) and radius 2.5.

### What is the general form of the equation of a circle?

x² + y² + Dx + Ey + F = 0. The centre is (−D/2, −E/2) and the radius is √(D²/4 + E²/4 − F). For x² + y² − 2x − 2y − 23 = 0 the centre is (1, 1) and the radius √(1 + 1 + 23) = 5. If D²/4 + E²/4 − F is zero the equation describes a single point, and if it is negative, no real circle.

### Why is there no circle through three points on a straight line?

A circle meets a straight line at most twice, so it cannot pass through three collinear points. In the formula, x₁(y₂ − y₃) + x₂(y₃ − y₁) + x₃(y₁ − y₂), which is twice the triangle's signed area, becomes zero and the centre would need a division by zero. Points that are nearly collinear give a very large radius.

### Quanto è preciso «Circonferenza per tre punti: centro, raggio, equazione»?

La precisione dipende dai dati inseriti e dalle ipotesi del metodo. Il calcolo decimale usa 50 cifre significative, ma stime, metodi numerici e dati di origine possono essere meno precisi; l’arrotondamento visualizzato non elimina questi limiti. Esempi svolti verificati con fonti indipendenti: 4. Per esempio, «(−3, 4), (4, 5), (1, −4)» viene verificato con Python 3.8 fractions with the determinant formula; each point is 5 from (1, 1): 4² + 3², 3² + 4², 0² + 5².

### Da dove proviene il metodo?

Weisstein, E. W. “Circumcircle” — MathWorld (circle through three points, determinant form); Weisstein, E. W. “Circle” — MathWorld (standard and general equations).

## Fonti

- [Weisstein, E. W. “Circumcircle” — MathWorld (circle through three points, determinant form)](https://mathworld.wolfram.com/Circumcircle.html)
- [Weisstein, E. W. “Circle” — MathWorld (standard and general equations)](https://mathworld.wolfram.com/Circle.html)