Question: how to factorise quartic equations?
Answer: If the quartic equation is also a difference of squares; then it can be factored just like a difference of squares can be factored. A difference of squares comes in the form of a2-b2 and factors like (a+b)(a-b). For example x4-81=0 would factor like (x2+9)(x2-9)=0. The (x2-9) will factor again to (x-3)(x+3).