In which year did Leonhard Euler introduced the symbol "i" to represent the square root of -1 ?

in 1555
208
in 1666
659
in 1777
1163
in 1888
658
The letter "i" as a designation of the element from -1 was proposed by Leonhard Euler in 1777. Later, from 1801, this designation was popularized by Carl Friedrich Gauss. Currently, it is the most commonly used symbol of the imaginary unit.
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