# 13 13 21 triangle

### Obtuse isosceles triangle.

Sides: a = 13   b = 13   c = 21

Area: T = 80.4810976013
Perimeter: p = 47
Semiperimeter: s = 23.5

Angle ∠ A = α = 36.12989274719° = 36°7'44″ = 0.63105687396 rad
Angle ∠ B = β = 36.12989274719° = 36°7'44″ = 0.63105687396 rad
Angle ∠ C = γ = 107.7422145056° = 107°44'32″ = 1.88804551744 rad

Height: ha = 12.38216886174
Height: hb = 12.38216886174
Height: hc = 7.66548548584

Median: ma = 16.21095650774
Median: mb = 16.21095650774
Median: mc = 7.66548548584

Inradius: r = 3.42547223835
Circumradius: R = 11.02443444346

Vertex coordinates: A[21; 0] B[0; 0] C[10.5; 7.66548548584]
Centroid: CG[10.5; 2.55549516195]
Coordinates of the circumscribed circle: U[10.5; -3.35994895762]
Coordinates of the inscribed circle: I[10.5; 3.42547223835]

Exterior(or external, outer) angles of the triangle:
∠ A' = α' = 143.8711072528° = 143°52'16″ = 0.63105687396 rad
∠ B' = β' = 143.8711072528° = 143°52'16″ = 0.63105687396 rad
∠ C' = γ' = 72.25878549439° = 72°15'28″ = 1.88804551744 rad

# How did we calculate this triangle?

Now we know the lengths of all three sides of the triangle and the triangle is uniquely determined. Next we calculate another its characteristics - same procedure as calculation of the triangle from the known three sides SSS. ### 1. The triangle circumference is the sum of the lengths of its three sides ### 2. Semiperimeter of the triangle ### 3. The triangle area using Heron's formula ### 4. Calculate the heights of the triangle from its area. ### 5. Calculation of the inner angles of the triangle using a Law of Cosines ### 6. Inradius ### 7. Circumradius ### 8. Calculation of medians #### Look also our friend's collection of math examples and problems:

See more informations about triangles or more information about solving triangles.