(1) x4−9x2+20=(x2−5)(x2−4)=(x−5)(x+5)(x−2)(x+2)x^4 - 9x^2 + 20 = (x^2-5)(x^2-4) = (x-\sqrt{5})(x+\sqrt{5})(x-2)(x+2)x4−9x2+20=(x2−5)(x2−4)=(x−5)(x+5)(x−2)(x+2) (2) x4−x2−6=(x2−3)(x2+2)=(x−3)(x+3)(x2+2)x^4 - x^2 - 6 = (x^2-3)(x^2+2) = (x-\sqrt{3})(x+\sqrt{3})(x^2+2)x4−x2−6=(x2−3)(x2+2)=(x−3)(x+3)(x2+2) (3) x4+3x2+9=(x2+3)2−3x2=(x2−3x+3)(x2+3x+3)x^4 + 3x^2 + 9 = (x^2+3)^2 - 3x^2 = (x^2-\sqrt{3}x+3)(x^2+\sqrt{3}x+3)x4+3x2+9=(x2+3)2−3x2=(x2−3x+3)(x2+3x+3) (4) x4+4y4+2x2y2=(x2+2y2)2−2x2y2=(x2−2xy+2y2)(x2+2xy+2y2)x^4 + 4y^4 + 2x^2y^2 = (x^2+2y^2)^2 - 2x^2y^2 = (x^2-\sqrt{2}xy+2y^2)(x^2+\sqrt{2}xy+2y^2)x4+4y4+2x2y2=(x2+2y2)2−2x2y2=(x2−2xy+2y2)(x2+2xy+2y2)