Question

Which of the following expressions is equal to -x^2 - 81? A. (-x-9i)(x+9i) B. (-x+9i)(x-9i) C. (-x-9i)(x-9i) D. (x+9i)(x-9i)

Answer 1

I think C. (-x-9i)(x-9i)

You can multiply them to get:
(-x-9i)(x-9i)=
`- x^(2) +9xi-9xi+81 i^(2)`=
remembering that:
`i= √(-1)` so that:

`i^(2) =-1`
Finally:

`- x^(2)+81 i^(2)`=
`= -x^(2) -81`