# 3 12 13 triangle

### Obtuse scalene triangle.

Sides: a = 3   b = 12   c = 13

Area: T = 17.55499287748
Perimeter: p = 28
Semiperimeter: s = 14

Angle ∠ A = α = 13.00328244668° = 13°10″ = 0.2276942099 rad
Angle ∠ B = β = 64.15875871263° = 64°9'27″ = 1.12197611355 rad
Angle ∠ C = γ = 102.84395884069° = 102°50'23″ = 1.79548894191 rad

Height: ha = 11.76999525165
Height: hb = 2.92549881291
Height: hc = 2.76999890423

Median: ma = 12.42197423484
Median: mb = 7.28801098893
Median: mc = 5.85223499554

Vertex coordinates: A[13; 0] B[0; 0] C[1.30876923077; 2.76999890423]
Centroid: CG[4.76992307692; 0.98999963474]
Coordinates of the circumscribed circle: U[6.5; -1.4811487494]
Coordinates of the inscribed circle: I[2; 1.25435663411]

Exterior (or external, outer) angles of the triangle:
∠ A' = α' = 166.99771755332° = 166°59'50″ = 0.2276942099 rad
∠ B' = β' = 115.84224128737° = 115°50'33″ = 1.12197611355 rad
∠ C' = γ' = 77.16604115931° = 77°9'37″ = 1.79548894191 rad

# How did we calculate this triangle?

We know the lengths of all three sides of the triangle, so the triangle is uniquely specified.

### 2. Semiperimeter of the triangle

The semiperimeter of the triangle is half its perimeter. The semiperimeter frequently appears in formulas for triangles to be given a separate name. By the triangle inequality, the longest side length of a triangle is less than the semiperimeter.

### 3. The triangle area using Heron's formula

Heron's formula gives the area of a triangle when the length of all three sides is known. There is no need to calculate angles or other distances in the triangle first. Heron's formula works equally well in all cases and types of triangles.

### 4. Calculate the heights of the triangle from its area.

There are many ways to find the height of the triangle. The easiest way is from the area and base length. The triangle area is half of the product of the base's length and height. Every side of the triangle can be a base; there are three bases and three heights (altitudes). Triangle height is the perpendicular line segment from a vertex to a line containing the base.

### 5. Calculation of the inner angles of the triangle using a Law of Cosines

The Law of Cosines is useful for finding a triangle's angles when we know all three sides. The cosine rule, also known as the Law of Cosines, relates all three sides of a triangle with an angle of a triangle. The Law of Cosines extrapolates the Pythagorean theorem for any triangle. Pythagorean theorem works only in a right triangle. Pythagorean theorem is a special case of the Law of Cosines and can be derived from it because the cosine of 90° is 0. It is best to find the angle opposite the longest side first. With the Law of Cosines, there is also no problem with obtuse angles as with the Law of Sines because the cosine function is negative for obtuse angles, zero for right, and positive for acute angles. We also use inverse cosine called arccosine to determine the angle from the cosine value.

An incircle of a triangle is a tangent circle to each side. An incircle center is called an incenter and has a radius named inradius. All triangles have an incenter, and it always lies inside the triangle. The incenter is the intersection of the three-angle bisectors. The product of the inradius and semiperimeter (half the perimeter) of a triangle is its area.