Trigonometry is the study of the relationship between the angles of triangles and their sides.
1957 to 1975 saw war between North and South Vietnam. Countries including the United States, Russia, and China, became involved. The country was reunified under a communist regime. The 1990's saw economic progress as a free-market system and trade with the West began again.
Back in the 1880's there was even a 'war of currents' between Thomas Edison (who helped invent DC) and Nikola Tesla (who helped invent AC). Both wanted their system to be used with AC eventually winning out due to the fact that it is safer and can be used over longer distances.
Between 1770 and 1780, Swedish pharmacist Carl Wilhelm Scheele, British clergyman Joseph Priestley and French chemist Antoine Laurent Lavoisier researched, documented and helped discover oxygen. The name oxygen was first used by Lavoisier in 1777.