A later unsuccessful attempt to prove the parallel postulate was proposed by the Persian mathematician *Nasir al-Din al-Tusi* (1201–1274). Al-Tusi wrote substantial works on astronomy and constructed the first modern observatory. He also wrote influential treatises on astronomy, logic, theology and ethics, and his extensive investigations into plane and spherical trigonometry included the sine rule for triangles.

*[Azerbaijan 2001; Iran 1956; Syria 2000]*