Respuesta :

Answer:

last option is correct

Step-by-step explanation:

(a-9)(a+9)

a(a+9)-9(a+9)

a^2+9a-9a-81  (plus 9a and minus -9a gets cancel)

a^2-81

a^2-9^2

Answer:

[tex]a^2-81[/tex]

Step-by-step explanation:

[tex](a-9)(a+9)[/tex]

Distribute, multiply everything in one of the parenthesis by everything in the other parenthesis,

[tex](a - 9)(a + 9)\\\\(a)(a)+(-9)(a)+(9)(a)+(-9)(9)[/tex]

Simplify, perform the multiplication and combine like terms,

[tex]a^2-9a+9a-81\\\\a^2-81[/tex]