Equation of the form `ax^4+bx^2+c=0`, where `a!=0` is called biquadratic. Biquadratic equations is solved with method of introducing new variable: setting `x^2=y`, we obtain quadratic equation `ay^2+by+c=0`.
Example. Solve equation `x^4+4x^2-21=0`.
Let `x^2=y`, then we obtain quadratic equation `y^2+4y-21=0`. Solutions of this equation are x=-7 and x=3.
Recall, that `x^2=y`, therefore we have set of equations: `x^2=-7,x^2=3`.
First equation doesn't have roots, second has two roots: `sqrt(3)` and `-sqrt(3)`. These two roots will be roots of the initial equation.