Question

What are the solutions of the equation `9x^4-2x^2-7=0`?

Answer 1

`x_{1,2}=pm1`

Explanation:

Writing
`x^4-2/9x^2-7/9=0`
and substituting
`x^2=t`
we get
`t^2-2/9t-7/9=0`
so
`t_1=1/9+8/9=1`
`t_2=-7/9<0`( which is impossible!)
so
`x^2=1`
`x_1=1`
`x_2=-1`