People can already count since several millennia. Arithmetic is improved over time. Usually we count by using integers. If we want to get more accurate results, then we better use rational numbers. Sometimes we also use numbers that are not expressible in a fraction, such as the number π (pi) and the square root of two . These are real numbers. The square root of a negative number delivers more problems. For this task it is necessary that we move to a two-dimensional number system. The better calculators under us can also handle this problem fluently. The so-called complex numbers are discovered in the sixteenth century by the Italian mathematician Gerolamo Cardano and are now used for all kinds of applications.
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