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In trigonometry, the law of tangents〔See Eli Maor, ''Trigonometric Delights'', Princeton University Press, 2002.〕 is a statement about the relationship between the tangents of two angles of a triangle and the lengths of the opposing sides. In Figure 1, ''a'', ''b'', and ''c'' are the lengths of the three sides of the triangle, and α, β, and γ are the angles ''opposite'' those three respective sides. The law of tangents states that : The law of tangents, although not as commonly known as the law of sines or the law of cosines, is equivalent to the law of sines, and can be used in any case where two sides and the included angle, or two angles and a side, are known. The law of tangents for spherical triangles was described in the 13th century by Persian mathematician Nasir al-Din al-Tusi (1201–74), who also presented the law of sines for plane triangles in his five-volume work ''Treatise on the Quadrilateral''.〔 〕〔 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Law of tangents」の詳細全文を読む スポンサード リンク
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