Question

Which of the following expressions is equal to -x^2-81

Answer 1

The answer is \(\Large \color{blue}{D.(-x-9i)(x-9i)}\)

Answer 2

Use: \(\Large \color{red}{a^2-b^2=(a+b)(a-b)}\) and \(\Large \color{red} {i^2=-1}\)
\(\Large \color{blue}{(-x-9i)(x-9i)=[(-9i)+(-x)][(-9i)-(-x)]\\=(-9i)^2-(-x)^2=81i^2-x^2=-81-x^2}\)

Answer 3

Thank you so much!

Answer 4

ur welcome ^^