⑴x^4+3x^2+4
=(x^+2)^2-x^2
=[(x^+2)+x][(x^2+2)-x]
=(x^2+x+2)(x^2-x+2)
⑵x^4-6x^2+1
=(x^2-1)^2-4x2
=[(x^2-1)+2x][(x^2-1)-2x]
=(x^2+2x-1)(x^2-2x-1)
⑶x^4-18x^2y^2+y^4
=(x^2-y^2)^2-16x^2y^2
=[(x^2-y^2)+4xy][(x^2-y^2)-4xy]
=(x^2+4xy-y^2)(x^2-4xy-y^2)
⑷x^4+4y^4
=(x^2+2y^2)^2-4x^2y^2
=[(x^2+2y^2)+2xy][(x^2+2y^2)-2xy]
=(x^2+2xy+2y^2)(x^2-2xy+2y^2)