Sine and cosine rules for triangles
Side-angle laws beyond right triangles.
Key knowledge
In a nondegenerate Euclidean triangle, a/sin A=b/sin B=c/sin C, where each side faces its named angle. The cosine rule gives c²=a²+b²−2ab cos C. The corresponding formulas hold after consistent relabeling.
Values and equations
a/sin A=b/sin B=c/sin C; c²=a²+b²−2ab cos C
- a, b, c
- Side lengths opposite A, B, C respectively same length unit
- A, B, C
- Interior angles; C lies between sides a and b rad
Conditions and scope
- Sides are positive lengths in a common unit; A,B,C are interior angles between 0 and π radians.
- Two sides and a nonincluded angle can permit two triangles; inverse sine alone may miss the supplementary angle.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Side-angle laws beyond right triangles.
2026-10-10 · Independent Physics and Measurement reference editor (AI editorial bounded review). Exact bounded introductory reference claims and this practical concept inventory are approved after closure. Existing university/research proof and source scopes remain separate; no universal knowledge-completeness or experimental-validation assertion.
Foundational review
2026-10-10 · Independent AI editorial inspection of this new compact reference entry, its equations, applicability, sources and knowledge connections.
Sources from other places
Agreement within this scope
Euclidean triangle sine and cosine rules, including a two-solution example.
Degrees versus radians do not change the relations when used consistently; the ambiguous example is not a complete classification.
Siyavula Education and Everything Maths textbook contributors
- Mathematics XI · Sine and cosine rules South AfricaEnglish
§6.5 Sine rule; Worked example 25; Ambiguous case; Cosine rule
- Mathematics XI · Sine and cosine rules
Entry details
Permanent link to this entry ↗- Record identity
- foundation-math-triangle-sine-cosine-rules
- Revision
- foundations-audit.1
- Structured reference record
- Foundational core completion