How do you factor a^6 + 7a^3 + 6?

2 Answers
Apr 15, 2016

(a + 1)(a^2 - a + 1)(a^3 + 6)

Explanation:

Call x = a^3. Factor the quadratic equation:
f(x) = x^2 + 7x + 6.
Since a - b + c = 0, use shortcut. One factor is (x + 1) and the other is (x + c/a) = (x + 6). We get:
f(x) = (x + 1)(x + 6)
Replace x by a^3.
f(a) = (a^3 + 1)(a^3 + 6)
Factor (a^3 + 1) and (a^3 + 6) by applying the algebraic identity:
a^3 + b^3 = (a + b)(a^2 - ab + b^2).
Finally,
f(a) = (a + 1)(a^2 - a + 1)(a^3 + 6)

Apr 15, 2016

a^6+7a^3+6=(a+1)(a^2-a+1)(a+root(3)(6))(a-root(3)(6)a+root(3)(36))

Explanation:

First treat this as a quadratic in a^3, noting that 1+6=7 and 1*6=6.

So:

a^6+7a^3+6 = (a^3)^2+7(a^3)+6 = (a^3+1)(a^3+6)

Note that both a^3 and 1=1^3 are perfect cubes, so we can use the sum of cubes identity:

A^3+B^3 = (A+B)(A^2-AB+B^2)

with A=a and B=b to find:

(a^3+1) = (a^3+1^3) = (a+1)(a^2-a+1)

The factor (a^3+6) is not quite so nice, but it can still be treated as a sum of cubes, using A=a and B=root(3)(6) as follows:

(a^3+6)

= (a^3+(root(3)(6))^3)

= (a+root(3)(6))(a-root(3)(6)a+(root(3)(6))^2)

= (a+root(3)(6))(a-root(3)(6)a+root(3)(6^2))

= (a+root(3)(6))(a-root(3)(6)a+root(3)(36))

Putting this together:

a^6+7a^3+6=(a+1)(a^2-a+1)(a+root(3)(6))(a-root(3)(6)a+root(3)(36))

This is as far as we can go with Real numbers.