Question

Why do we use $i$ in math?

Answer 1

$i$ or iota is an imaginary number.

It denotes the square root of negative $1$. This means $i=\sqrt{-1}$

In mathematics, $i$ is used in complex numbers.

Example:

$z=2+3i$ is a complex number.

where $3i$ is an imaginary part of a complex number