"R" is the18th letter in the English alphabet.
R evolved from the 20th pictographic letter of Phoenicia. The Phoenicians called it resh, which means "head". Since ancient Rome, R has been called Littera can and na (Latin, the letter of dog, translated from English into the letter of dog or the letter of growling), because the pronunciation of R is quite similar to that of a dog howling R-R-R or GR-R-R.
Extended data
The use of "r" in various industries;
1, mathematics
(1) number theory? r？ Represents the set of real numbers in set theory, and the real part of complex numbers is also represented by this symbol.
(2) r of geometry? Or? r？ Represents the radius of a circle.
2. In electromagnetism, italics? r？ Stands for resistance, and positive R stands for live wire.
3. Optical R is a unit for measuring ionizing radiation of fdg by X-ray.
4. Chemistry
(1) in gg physical chemistry, r stands for molar gas constant (universal gas constant) r = 8.314510J mol-1k-1.
(2) R in organic chemistry is the floorboard of hydrocarbon chain.
(3) In stereochemistry, R represents the right-handed (clockwise) molecular arrangement near a carbon atom, and the judgment method is based on the priority rule of Cahn Ingold Prelog.
(4) Biochemical R stands for arginine.
(5) R of chemical reaction? Represents the molar power generation per unit capacity.