中四數學題

x的5次方-8x的3次方-9x
=x^5-8x^3-9x
=x(x^4-8x^2-9)
=x(x^2-9)(x^2+1)
=x(x+3)(x-3)(x^2+1)

x^5-8x^3-9x=0
x(x^4-8x^2-9)=0
=>x=0
x^4-8x^2-9=0
(x^2-9)(x^2+1)=0
=>x^2=9
=x=3, x=-3
i don't remember do you have complex number, if no just reject (x^2=-1),
if yes x^(2)=-1, x=i, x=-i

i.e. The roots are x=0, 3, -3, i, -i.