How do you factor the expression #x^8 - 1#?

1 Answer
George C.
Dec 20, 2015

Use some identities to find:

#x^8-1 = (x-1)(x+1)(x^2+1)(x^2+sqrt(2)x+1)(x^2-sqrt(2)x+1)#

Explanation:

We shall use some identities to help:

The difference of squares identity:

#a^2-b^2 = (a-b)(a+b)#

Consider also:

#(a^2+kab+b^2)(a^2-kab+b^2)=a^4+(2-k^2)a^2b^2+b^4#

In particular if #k=sqrt(2)# then:

#(a^2+sqrt(2)ab+b^2)(a^2-sqrt(2)ab+b^2) = a^4+b^4#

So:

#x^8-1#

#=(x^4)^2-1^2#

#=(x^4-1)(x^4+1)#

#=((x^2)^2-1^2)(x^4+1)#

#=(x^2-1)(x^2+1)(x^4+1)#

#=(x^2-1^2)(x^2+1)(x^4+1)#

#=(x-1)(x+1)(x^2+1)(x^4+1)#

#=(x-1)(x+1)(x^2+1)(x^4+1^4)#

#=(x-1)(x+1)(x^2+1)(x^2+sqrt(2)x+1)(x^2-sqrt(2)x+1)#

This is as far as we can go with Real coefficients. The remaining quadratic factors all have Complex zeros.

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