"X" is a British mathematician William? Oughtred proposed in 163 1 that in his works, he used "x" to represent multiplication. If the "+"sign is a way to indicate the quantity increase, then the "×" sign is a way to indicate the quantity increase faster, so the "+"sign is written obliquely. After the appearance of "X", it was resolutely opposed by the German mathematician Leibniz on the grounds that "X" was similar to the Latin letter "X" and was easily confused. Leibniz is in favor of using "?" Stands for multiplication. 1637, the French mathematician Descartes also adopted "?" Symbols represent multiplication, symbols "×" and "?" Symbols are deadlock, and these two operation symbols have been used until today. Leibniz once proposed to use "∩" to represent multiplication, and this symbol is now mainly used in set theory to represent the intersection of sets. If A represents the set of all isosceles triangles and B represents the set of all right-angled triangles, then their intersection A∩B is the set of all right-angled isosceles triangles.

Besides, "?" And "x" can also describe the dot product and cross product of two vectors A and B. If |a|=a, |b|=b and the included angle (a, b)=θ, then a? B=abcosθ, a×b=(absin θ)c0, where c0 represents the unit vector perpendicular to the two vectors A and B, and the direction obeys the right-handed system.