Tiffany152 Tiffany152

25-05-2015
Mathematics

Answered

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 :

25-05-2015

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

You can multiply them to get:
(-x-9i)(x-9i)=[tex]- x^{2} +9xi-9xi+81 i^{2} [/tex]=
remembering that: [tex]i= \sqrt{-1} [/tex] so that:
[tex] i^{2} =-1[/tex]
Finally:
[tex]- x^{2}+81 i^{2} [/tex]=[tex]= -x^{2} -81[/tex]

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