Law of cosines

Figure 1 – A triangle. The angles α (or A), β (or B), and γ (or C) are respectively opposite the sides a, b, and c.

Fig. 2 – Obtuse triangle ABC with perpendicular BH

Fig. 3 – Applications of the law of cosines: unknown side and unknown angle.

In trigonometry, the law of cosines (also known as the cosine formula or cosine rule) relates the lengths of the sides of a triangle to the cosine of one of its angles. Using notation as in Fig. 1, the law of cosines states

where $γ$ denotes the angle contained between sides of lengths $a$ and $b$ and opposite the side of length $c$ .

单词	Cosine Rule
释义	Cosine Rule 英语百科 Law of cosines In trigonometry, the law of cosines (also known as the cosine formula or cosine rule) relates the lengths of the sides of a triangle to the cosine of one of its angles. Using notation as in Fig. 1, the law of cosines states where $γ$ denotes the angle contained between sides of lengths $a$ and $b$ and opposite the side of length $c$ .
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Cosine Rule

Law of cosines