The term "biquadratic equation" is sometimes used to as a synonym for quartic equation (Beyer 1987b, p. 34), but
perhaps more commonly (e.g., Hazewinkel 1988; Gellert et al. 1989, p. 101)
and more properly for a quartic equation having no odd powers, i.e.,
Such equations are easy to solve, since they reduce to a quadratic equation in the variable and hence can be solved for using the quadratic formula
and then in terms of the original variables by taking the square
roots.
and hence can be solved for 
using the quadratic formula
and then in terms of the original variables by taking the square
roots.
