# 18 18 21 triangle

### Acute isosceles triangle.

Sides: a = 18   b = 18   c = 21

Area: T = 153.5122010931
Perimeter: p = 57
Semiperimeter: s = 28.5

Angle ∠ A = α = 54.31546652873° = 54°18'53″ = 0.94879697414 rad
Angle ∠ B = β = 54.31546652873° = 54°18'53″ = 0.94879697414 rad
Angle ∠ C = γ = 71.37106694253° = 71°22'14″ = 1.24656531708 rad

Height: ha = 17.05768901034
Height: hb = 17.05768901034
Height: hc = 14.62201915172

Median: ma = 17.36437553542
Median: mb = 17.36437553542
Median: mc = 14.62201915172

Inradius: r = 5.38663863484
Circumradius: R = 11.08105662025

Vertex coordinates: A[21; 0] B[0; 0] C[10.5; 14.62201915172]
Centroid: CG[10.5; 4.87333971724]
Coordinates of the circumscribed circle: U[10.5; 3.54396253147]
Coordinates of the inscribed circle: I[10.5; 5.38663863484]

Exterior(or external, outer) angles of the triangle:
∠ A' = α' = 125.6855334713° = 125°41'7″ = 0.94879697414 rad
∠ B' = β' = 125.6855334713° = 125°41'7″ = 0.94879697414 rad
∠ C' = γ' = 108.6299330575° = 108°37'46″ = 1.24656531708 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.