二次方程......

9x^4-40x^2+16=0
let a=x^2, we have
9a^2-40a+16=0
(9a-4) (a-4)=0
a=4/9 or a=4
since a=x^2
therefore x=2/3 or x=2

y=x^2,
9y-40+16=0
(9y-4)(y-4)=0
y=4/9 or 4
As y=x^2,
x= 2/3 or -2/3 or 2 or -2