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

1 Answer

May 18, 2016

#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)#

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