Triangles

Perimeter of triangle
P = a + b + c
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Semiperimeter of triangle
p = (a + b + c) / 2
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Cosine rule
c^2 = a^2 + b^2  - 2ab cos(α)
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Law of sines (sine rule)
a / sin(A) = b / sin(B)
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Law of sines (sine rule)
a / sin(A) = 2 R
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Angle bisector theorem
m / n  = b / c
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Bisector (bisectrix) of triangle
l = saknis(bc - mn)
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Length of triangle median
m_a = 1/2 saknis(2 (b^2 + c^2) - a^2)
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Inradius of triangle
r = S / p
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Circumradius of a triangle
R = abc / 4S
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Pythagorean theorem
c^2 = a^2 + b^2
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Inradius of right (angled) triangle
r = (a + b - c) / 2
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Circumradius of a right (angled) triangle
R = c / 2
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Circumradius of a equilateral triangle
R = a saknis(3) / 3
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Inradius of a equilateral triangle
r = a saknis(3) / 6
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