Question

X^4-10x^2+9 how do I factor this?

Answer 1

X^2 = y and rewrite your exercise 

y^2 -10y +9 = y*y -y -9y +9 = y(y -1) -9(y -1) = (y -9)(y -1) 
and now rewrite using this above wrote note of x^2 = y 

so will get (y -9)(y -1) = (x^2 -9)(x^2 -1)

and this mean that 

x^4 -10x^2 +9 = (x^2 -9(x^2 -1)

hope helped