Question

1/√-2.75

Answer 1

That requires complex numbers since its the square root of a negative number and there are no real numbers that when squared produce a negative number.
\[1/ \sqrt{-2.75} = 1/ i \sqrt{2.75}\]

Answer 2

i is what?

Answer 3

i is defined to be the square root of -1.

Answer 4

and is writen as just i or what?

Answer 5

Yep it's just written as i. You can further simplify the expression by multiplying the expression by \[-i \sqrt{2.75} /-i \sqrt{2.75} \] since that is equal to 1 so doesn't change the value of the expression.