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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)

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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


User Nuno Henriques
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