Islamic culture has significantly progressed the development of trigonometric identities used in a modern context. In Islamic culture, the origin of trigonometry comes from its significant importance in identifying the exact times for religious practices through astronomy. Greek astronomers influenced the study of trigonometry in Islamic culture through Ptolemy's Almagest. Ptolemy's work reached Islamic society and was received with great approval, praise and curiosity, launching their studies for trigonometry significantly. This can be seen through the title of Ptolemy's work itself, originally titled The Great Arrangement was rendered and translated into Al-Majest meaning The Greatest. Additionally, through Islamic culture, Almagest reached European scholars and cultures, in which the study evolved even further. 

The 13th-century astronomer whose work The Transversal Figure explained how the length of chord tables in Ptolemy's Almagest could be utilised in right-angled triangles.  Al Tusi linked the arcs of a circle and triangles, concluding that countless types of triangles could fit within a circle, and therefore be considered chords that subtend the arcs on the opposite side of the angles within the triangle. By doing so, mathematicians' reliance on tables decreased significantly, allowing for further development of trigonometric identities to be discovered. 

In one of his works, the Kitab al-Kamil, he presented a simplified version of the Almagest in which he created his sine table accurate up to 8 decimal places and a table of tangent values. In his work in the 10th century AD, he employed all six trigonometric identities (sine, cosine, tangent, secant, cosecant, cotangent) and he developed the double angle identity by manipulating the sum of angles identity. In his instructions, he states to "...multiply the sine by the cosine minutes, and the result is half the sine of the double."

Figure g: sundial