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How do you factor $x^{2} - 81$ $x^2-81$ ?

1 Answer

May 18, 2016

$x^{2} - 81 = (x - 9) (x + 9)$ $x^2-81=color(green)((x-9)(x+9))$

Explanation:

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

$(x^{2} - 81) = (x^{2} - 9^{2})$ $(x^2-81 ) = (x^2-9^2)$

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

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