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5 天前 · Nitrogen is a chemical element; it has symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at seventh in total abundance in the Milky Way and the Solar System.
1 天前 · Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the formula N H 3. A stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pungent smell.
1 天前 · Probability theory. In probability theory and statistics, a normal distribution or Gaussian distribution is a type of continuous probability distribution for a real-valued random variable. The general form of its probability density function is The parameter is the mean or expectation of the distribution (and also its median and mode ), while ...
3 天前 · He is traditionally said to have banned and burned many books and executed scholars. His public works projects included the incorporation of diverse state walls into a single Great Wall of China and a massive new national road system, as well as his city-sized mausoleum guarded by a life-sized Terracotta Army.
4 天前 · This is a list of the tallest buildings. Tall buildings, such as skyscrapers, are intended here as enclosed structures with continuously occupiable floors and a height of at least 350 metres (1,150 ft). Such definition excludes non-building structures, such as towers .
1 天前 · Middle Ages - Wikipedia. A stained-glass panel from Canterbury Cathedral, c. 1175 – c. 1180. It depicts the Parable of the Sower, a biblical narrative. In the history of Europe, the Middle Ages or medieval period (also spelt mediaeval or mediæval) lasted from approximately 500 to 1500 AD.
3 天前 · The speed at which the individual crests and troughs of a plane wave (a wave filling the whole space, with only one frequency) propagate is called the phase velocity vp. A physical signal with a finite extent (a pulse of light) travels at a different speed.
