To solve this problem, we must first understand what a math textbook refers to. Mathematics textbook refers to mathematics textbook. Different mathematics textbooks have different lengths. The length of the math textbook we use is 30 cm, which is 3 cm.
Extended data:
In the international system of units, the standard unit of length is "meter", which is represented by the symbol "m". 1960 The 1st International Metrology Conference: "The length of the meter is equal to 1650763.73 times of the radiation wavelength passing between 2P 10 and 5d 1 levels of krypton -86 atoms in vacuum".
Generally speaking, "meter" is generally defined as the length of the distance traveled by light in the time interval of 1/299792458 s in vacuum. The unit of length adopted in China is consistent with the international system of units, that is, "meter" is the legal unit of length measurement in China.