In mathematics, imaginary numbers are numbers in the form of a+b*i, where a and b are real numbers, b≠0, i? = - 1。 The word imaginary number was founded by Descartes, a famous mathematician in the17th century, because the concept at that time thought it was a non-existent real number. Later, it was found that the real part A of the imaginary number a+b*i can correspond to the horizontal axis and the imaginary part B can correspond to the vertical axis on the plane, so that the imaginary number a+b*i can correspond to the points (a, b) on the plane.
The imaginary number bi can be added to the real number A to form a complex number in the form of a+bi, where the real numbers A and B are called the real part and imaginary part of the complex number respectively. Some authors use the term pure imaginary number to represent the so-called imaginary number, which refers to any complex number whose imaginary part is not zero.
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1777, the Swiss mathematician Euler (or translated as Euler) began to use the symbol I to represent imaginary units. Then people organically combine imaginary numbers with real numbers and write them in the form of a+bi (A and B are both real numbers, when A equals 0, they are pure imaginary numbers, when ab is not equal to 0, they are complex numbers, and when B equals 0, they are real numbers).
In engineering operation, in order not to be confused with other symbols (such as the symbol of current), letters such as J or K are sometimes used to represent imaginary units. Usually, we use the symbol C to represent the complex set and the symbol R to represent the real set.