Question

How do you factor `x^2-81`?

Answer 1

`x^2-81=color(green)((x-9)(x+9))`

Explanation:

For the general case:
`color(white)("XXX")(x^2-a^2)=(x-a)(x+a)`

`(x^2-81 ) = (x^2-9^2)`

substituting `9` for `a` in the general case gives:
`color(white)("XXX")(x-9)(x+9)`