The Law of Tangents is a rather obscure trigonometric identity that is sometimes used in place of its better-known counterparts, the law of sines and law of cosines, to calculate angles or sides in a triangle.
According to Wikipedia,the law of tangents for planar triangles was described in the 11th century by Ibn Muʿādh al-Jayyānī (Wikipedia cites Roshdi Rashed (ed.) Encyclopedia Of The History Of Arabic Science, p. 182. But I couldn't find any reference to the law of tangents in this book). Another source credits the Persian mathematician Abu'l-Wafa' in the 10th century CE as the first to publish it.
The following statement can be found in Merzbach and Boyer, A History of Mathematics, 3rd edition, page 278:
"Though Viete may have been the first to use this formula [the law of tangents], it was first published by the German physician and professor of mathematics Thomas Finck (1561-1656) in 1583, in Geometriae Rotundi Libri XIV."
Who was really the first to publish (or whatever that means back then) it?