Triangles

Perimeter of triangle
P = a + b + c

Semiperimeter of triangle
p = (a + b + c) / 2

Cosine rule
c^2 = a^2 + b^2  - 2ab cos(α)

Law of sines (sine rule)
a / sin(A) = b / sin(B)

Law of sines (sine rule)
a / sin(A) = 2 R

Angle bisector theorem
m / n  = b / c

Bisector (bisectrix) of triangle
l = saknis(bc - mn)

Length of triangle median
m_a = 1/2 saknis(2 (b^2 + c^2) - a^2)

Inradius of triangle
r = S / p

Circumradius of a triangle
R = abc / 4S

Pythagorean theorem
c^2 = a^2 + b^2

Inradius of right (angled) triangle
r = (a + b - c) / 2

Circumradius of a right (angled) triangle
R = c / 2

Circumradius of a equilateral triangle
R = a saknis(3) / 3

Inradius of a equilateral triangle
r = a saknis(3) / 6