triangle

If now A, B, and C denote the angles of the triangle, and a, b, and c the lengths of the sides opposite them, the following results hold:

Proving the angle sum formula

Determining the area of a triangle

(i) The area of the triangle equals .

(ii) The sine rule:

where R is the radius of the circumcircle.

(iii) The cosine rule:

(iv) The tangent rule:

(v) Hero's formula: Let . Then the area of the triangle equals

See acute, obtuse, oblique, equilateral, isosceles triangle, right-angled triangle, scalene triangle, solution of triangles, spherical triangle.

• Kolmogorov–Smirnov test
• Kovalevskaya, Sofya
• Kraft inequality
• Kronecker delta
• Kronecker, Leopold
• Kronecker's Lemma
• Kruskal's algorithm
• Kruskal–Wallis test
• Kummer, Ernst
• Kuratowski closure axioms
• Kuratowski's Theorem
• kurtosis
• Königsberg bridge problem
• k‐simplex
• L
• labelling algorithm
• lag
• Lagrange interpolation
• Lagrange, Joseph‐Louis
• Lagrange multiplier
• Lagrange point
• Lagrange's equations
• Lagrange's Theorem
• Lagrange's Theorem
• Lagrangian