Editing Quartic function (section)

====Biquadratic equation====
If {{math|''a''<sub>3</sub> {{=}} ''a''<sub>1</sub> {{=}} 0}} then the function 
:<math>
Q(x) = a_4x^4+a_2x^2+a_0
</math> 
is called a '''biquadratic function'''; equating it to zero defines a '''biquadratic equation''', which is easy to solve as follows

Let the auxiliary variable {{math|''z'' {{=}} ''x''<sup>2</sup>}}.
Then {{math|''Q''(''x'')}} becomes a [[Quadratic function|quadratic]] {{math|''q''}} in {{math|''z''}}: {{math|''q''(''z'') {{=}} ''a''<sub>4</sub>''z''<sup>2</sup> + ''a''<sub>2</sub>''z'' + ''a''<sub>0</sub>}}. Let {{math|''z''<sub>+</sub>}} and {{math|''z''<sub>−</sub>}} be the roots of {{math|''q''(''z'')}}. Then the roots of the quartic {{math|''Q''(''x'')}} are
:<math>
\begin{align}
x_1&=+\sqrt{z_+},
\\
x_2&=-\sqrt{z_+},
\\
x_3&=+\sqrt{z_-},
\\
x_4&=-\sqrt{z_-}.
\end{align}
</math>