# Rechtwinkliges Dreieck: Hypotenuse, Seiten und Winkel

> Hypotenuse, fehlende Kathete, spitze Winkel, Fläche und Umfang aus zwei Werten mit dem Satz des Pythagoras und Trigonometrie berechnen.

Interaktive Version: https://www.calcopenly.com/de/geometry/right-triangle-calculator
Thema: Geometrierechner

A right triangle is fixed by any two of its parts as long as one of them is a side. With both legs known, Pythagoras' theorem gives the hypotenuse c = √(a² + b²). With a side and an acute angle, the rest follows from sin α = a/c, cos α = b/c and tan α = a/b, and the two acute angles always add up to 90°.

The default legs of 3 cm and 4 cm make the 3-4-5 triangle: hypotenuse 5 cm, area 6 cm², perimeter 12 cm and angles of 36.8699° and 53.1301°. Builders use that ratio to square a corner, since marks 3 m and 4 m from it along two walls should be exactly 5 m apart. The results also include the height onto the hypotenuse (ab/c), the inradius ((a + b − c)/2) and the circumradius (c/2).

Angle α is opposite leg a and β is opposite leg b. Angles can be typed in degrees, radians, gradians or turns and must lie strictly between 0° and 90°.

## Eingaben

- **I know** (Optionen: Both legs (a and b), Leg a and hypotenuse c, Leg b and hypotenuse c, Leg a and angle α, Leg b and angle α, Hypotenuse c and angle α, Leg a and angle β, Leg b and angle β, Hypotenuse c and angle β)
- **Leg a (opposite α)**
- **Leg b (opposite β)**
- **Hypotenuse c**
- **Angle α**: Between 0° and 90°
- **Angle β**: Between 0° and 90°
- **Ergebniseinheit** (Optionen: Millimeter (mm), Zentimeter (cm), Meter (m), Kilometer (km), Zoll (in), Fuß (ft), Yard (yd), Meilen (mi))

## Ergebnisse

- Fläche — Hauptergebnis
- Leg a
- Leg b
- Hypotenuse c
- Angle α (°)
- Angle β (°)
- Umfang
- Height onto the hypotenuse
- Inradius
- Circumradius

## Formel

$$
a^2 + b^2 = c^2,\quad \sin\alpha = \frac{a}{c},\quad \cos\alpha = \frac{b}{c},\quad \tan\alpha = \frac{a}{b},\quad \alpha + \beta = 90^\circ
$$

## Durchgerechnete Beispiele

### Legs 3 and 4

- I know: Both legs (a and b)
- Leg a (opposite α): 3 cm
- Leg b (opposite β): 4 cm
- Ergebniseinheit: Zentimeter (cm)
- **Hypotenuse c: 5 cm**
- **Fläche: 6 cm²**
- **Angle α: 36.869898 °**
- **Angle β: 53.130102 °**
- **Height onto the hypotenuse: 2.4 cm**
- **Inradius: 1 cm**
- **Circumradius: 2.5 cm**
- **Umfang: 12 cm**
- Prüfquelle: 3-4-5 triple; h = 3·4/5; r = (3 + 4 − 5)/2; Python 3.8 math: degrees(atan(3/4))

### Leg 5 and hypotenuse 13

- I know: Leg a and hypotenuse c
- Leg a (opposite α): 5 cm
- Hypotenuse c: 13 cm
- Ergebniseinheit: Zentimeter (cm)
- **Leg b: 12 cm**
- **Fläche: 30 cm²**
- **Umfang: 30 cm**
- **Angle α: 22.619865 °**
- Prüfquelle: 5-12-13 triple; Python 3.8 math: degrees(asin(5/13))

### 30-60-90 from hypotenuse 10 and α = 30°

- I know: Hypotenuse c and angle α
- Hypotenuse c: 10 cm
- Angle α: 30 °
- Ergebniseinheit: Zentimeter (cm)
- **Leg a: 5 cm**
- **Leg b: 8.660254 cm**
- **Angle β: 60 °**
- **Fläche: 21.650635 cm²**
- Prüfquelle: Side opposite 30° is half the hypotenuse; Python 3.8 math: 10*cos(radians(30)), 25*sqrt(3)/2

### 45° angle gives equal legs (edge case)

- I know: Leg a and angle α
- Leg a (opposite α): 7 cm
- Angle α: 45 °
- Ergebniseinheit: Zentimeter (cm)
- **Leg b: 7 cm**
- **Hypotenuse c: 9.899495 cm**
- **Fläche: 24.5 cm²**
- **Angle β: 45 °**
- Prüfquelle: tan 45° = 1 so b = a; Python 3.8 math: 7*sqrt(2)

### Leg b = 9 and β = 60°

- I know: Leg b and angle β
- Leg b (opposite β): 9 cm
- Angle β: 60 °
- Ergebniseinheit: Zentimeter (cm)
- **Leg a: 5.196152 cm**
- **Hypotenuse c: 10.392305 cm**
- **Angle α: 30 °**
- Prüfquelle: Python 3.8 math: 9/tan(radians(60)), 9/sin(radians(60))

### Legs 6 in and 8 in, results in cm

- I know: Both legs (a and b)
- Leg a (opposite α): 6 in
- Leg b (opposite β): 8 in
- Ergebniseinheit: Zentimeter (cm)
- **Hypotenuse c: 25.4 cm**
- **Fläche: 154.8384 cm²**
- Prüfquelle: 6-8-10 in triangle; 10 in = 25.4 cm and 24 in² = 24 × 6.4516 cm² exactly (1 in = 2.54 cm)

## Fragen

### How do you find the hypotenuse of a right triangle?

Square both legs, add them and take the square root: c = √(a² + b²). Legs of 3 and 4 give √25 = 5, and legs of 6 in and 8 in give 10 in. If you know one leg and the angle opposite it instead, divide by the sine: c = a / sin α, so a 5 cm leg opposite 30° means a 10 cm hypotenuse.

### How do you find a missing side of a right triangle?

For a missing leg, subtract the squares: b = √(c² − a²). A hypotenuse of 13 and a leg of 5 give √(169 − 25) = 12. The hypotenuse must be longer than either leg, or no right triangle exists. With one side and an angle, use SOHCAHTOA: opposite = hypotenuse × sin α and adjacent = hypotenuse × cos α.

### What are the side ratios of a 30-60-90 and a 45-45-90 triangle?

A 30-60-90 triangle has sides in the ratio 1 : √3 : 2, so the side opposite 30° is half the hypotenuse; a hypotenuse of 10 gives legs of 5 and 8.6603. A 45-45-90 triangle has equal legs and a hypotenuse √2 ≈ 1.4142 times a leg, so legs of 7 give a hypotenuse of 9.8995.

### What does SOHCAHTOA mean?

It is a memory aid for the three trigonometric ratios in a right triangle: sine = opposite/hypotenuse, cosine = adjacent/hypotenuse, tangent = opposite/adjacent. In the 3-4-5 triangle, the angle opposite the side of 3 has sin = 0.6, cos = 0.8 and tan = 0.75, and each inverse function returns the same 36.87°.

### How do you check a corner is square with the 3-4-5 rule?

Mark 3 units along one side and 4 along the other, measured from the corner; the diagonal between the marks is exactly 5 when the angle is 90°. A longer diagonal means the angle is too wide: 5.05 m on 3 m and 4 m legs is 91.2°, and 4.95 m is 88.8°. Multiples such as 6-8-10 or 9-12-15 give a more precise check on large slabs.

### Wie genau arbeitet „Rechtwinkliges Dreieck: Hypotenuse, Seiten und Winkel“?

Die Genauigkeit hängt von Ihren Eingaben und den Annahmen der Methode ab. Die Dezimalrechnung nutzt 50 signifikante Stellen, doch Schätzungen, numerische Verfahren und Quelldaten können ungenauer sein. Die angezeigte Rundung beseitigt diese Grenzen nicht. Anhand unabhängiger Quellen geprüfte Rechenbeispiele: 6. Beispielsweise wird „Legs 3 and 4“ anhand von 3-4-5 triple; h = 3·4/5; r = (3 + 4 − 5)/2; Python 3.8 math: degrees(atan(3/4)) geprüft.

### Woher stammt die Methode?

Euclid, Elements, Book I, Proposition 47 (Pythagorean theorem); OpenStax Precalculus 2e, §5.4 Right Triangle Trigonometry.

## Quellen

- [Euclid, Elements, Book I, Proposition 47 (Pythagorean theorem)](https://mathcs.clarku.edu/~djoyce/java/elements/bookI/propI47.html)
- [OpenStax Precalculus 2e, §5.4 Right Triangle Trigonometry](https://openstax.org/books/precalculus-2e/pages/5-4-right-triangle-trigonometry)