9 18 21 triangle

Obtuse scalene triangle.

Sides: a = 9   b = 18   c = 21

Area: T = 80.498844719
Perimeter: p = 48
Semiperimeter: s = 24

Angle ∠ A = α = 25.20987652968° = 25°12'32″ = 0.44399759548 rad
Angle ∠ B = β = 58.41218644948° = 58°24'43″ = 1.01994793577 rad
Angle ∠ C = γ = 96.37993702084° = 96°22'46″ = 1.68221373411 rad

Height: ha = 17.889854382
Height: hb = 8.944427191
Height: hc = 7.667651878

Median: ma = 19.03328663107
Median: mb = 13.4166407865
Median: mc = 9.60546863561

Inradius: r = 3.35441019662
Circumradius: R = 10.56554211937

Vertex coordinates: A[21; 0] B[0; 0] C[4.71442857143; 7.667651878]
Centroid: CG[8.57114285714; 2.556550626]
Coordinates of the circumscribed circle: U[10.5; -1.17439356882]
Coordinates of the inscribed circle: I[6; 3.35441019662]

Exterior(or external, outer) angles of the triangle:
∠ A' = α' = 154.7911234703° = 154°47'28″ = 0.44399759548 rad
∠ B' = β' = 121.5888135505° = 121°35'17″ = 1.01994793577 rad
∠ C' = γ' = 83.62106297916° = 83°37'14″ = 1.68221373411 rad

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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.

a = 9 ; ; b = 18 ; ; c = 21 ; ;

1. The triangle circumference is the sum of the lengths of its three sides

p = a+b+c = 9+18+21 = 48 ; ;

2. Semiperimeter of the triangle

s = fraction{ o }{ 2 } = fraction{ 48 }{ 2 } = 24 ; ;

3. The triangle area using Heron's formula

T = sqrt{ s(s-a)(s-b)(s-c) } ; ; T = sqrt{ 24 * (24-9)(24-18)(24-21) } ; ; T = sqrt{ 6480 } = 80.5 ; ;

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

T = fraction{ a h _a }{ 2 } ; ; h _a = fraction{ 2 T }{ a } = fraction{ 2 * 80.5 }{ 9 } = 17.89 ; ; h _b = fraction{ 2 T }{ b } = fraction{ 2 * 80.5 }{ 18 } = 8.94 ; ; h _c = fraction{ 2 T }{ c } = fraction{ 2 * 80.5 }{ 21 } = 7.67 ; ;

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

a**2 = b**2+c**2 - 2bc cos( alpha ) ; ; alpha = arccos( fraction{ a**2-b**2-c**2 }{ 2bc } ) = arccos( fraction{ 9**2-18**2-21**2 }{ 2 * 18 * 21 } ) = 25° 12'32" ; ; beta = arccos( fraction{ b**2-a**2-c**2 }{ 2ac } ) = arccos( fraction{ 18**2-9**2-21**2 }{ 2 * 9 * 21 } ) = 58° 24'43" ; ; gamma = arccos( fraction{ c**2-a**2-b**2 }{ 2ba } ) = arccos( fraction{ 21**2-9**2-18**2 }{ 2 * 18 * 9 } ) = 96° 22'46" ; ;

6. Inradius

T = rs ; ; r = fraction{ T }{ s } = fraction{ 80.5 }{ 24 } = 3.35 ; ;

7. Circumradius

R = fraction{ a }{ 2 * sin( alpha ) } = fraction{ 9 }{ 2 * sin 25° 12'32" } = 10.57 ; ;




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