Factor the Expression completely? x^4+2^3-15x^2?

Question

Factor the Expression completely? x^4+2^3-15x^2?

Answer 1

x⁴+2³-15x²

x⁴ + 8 - 15x²

x² (x² - 15) + 8

x² (x + √15) (x - √15) + 8

.

Answer 2

x^2 (x^2 + 2x - 15)  Leaves you with a quadratic

x^2 (x-3)(x+5)  --->the 2 terms here can be gotten from the quadratic formula if need be, i didn't use it but it will prove it.

Edit:

Not sure why I got a thumbs down, but the guy below me is wrong.

proof of my method:

x^2 * (x^2 +5x -3x - 15) multiply out the 2 bracketed terms

x^2 * (x^2 + 2x - 15)

x^4 + 2x^3 - 15x^2 which matches the given unsimplified expression

The zero's of my simplification are 0, 3, -5

However from the other reply:

x^2(x+ root(15))(x - root(15)) + 8

  expand this out

x^2(x^2 -xroot15 + xroot15 - 15) + 8

x^2(x^2 - 15) + 8

x^4 -15x^2 + 8

x^4 - 15x^2 + 8 =/ x^4 + 2x^3 - 15x^2

If somehow the other poster is right I don't see it.