a. x^4 + 4y^4
= x^4 + 4x^2y^2 + 4y^4 - 4x^2y^2
= (x^2)^2 + 2.x^2.2y^2 + (2y^2)^2 - 4x^2y^2
= (x^2 + 2y^2)^2 - 4x^2y^2
<hđt số 1>
= (x^2 + 2y^2)^2 - 2^2x^2y^2
= (x^2 + 2y^2)^2 - (2xy)^2
= [(x^2 + 2y^2) - (2xy)].[(x^2 + 2y^2) + (2xy)]
= (x^2 + 2y^2 - 2xy).(x^2 + 2y^2 + 2xy)
<hđt số 3>

f. 81x^4 + 4
= 9^2(x^2)^2 + 2^2
= 9^2(x^2)^2 + 36.x^2 + 2^2 - 36.x^2
= (9x^2)^2 + 2.2.9x^2 + 2^2 - 36x^2
= (9x^2 + 2)^2 - 36x^2
<hđt số 1>
= [(9x^2 + 2) - (6x)].[(9x^2 + 2) + (6x)]
= (9x^2 + 2 - 6x).(9x^2 + 6x + 2)
<hđt số 3>

d.
4x^4y^4+1
= 4.(xy)^4 + 4.(xy)^2 + 1 - 4(xy)^2
= (2(xy)^2 + 1) - 4(xy)^2
= (2(xy)^2 - 2(xy) + 1)(2(xy)^2 + 2(xy) + 1)
e.
64x^4+y^4
= (8x^2)^2 + 2.(8x^2).y^2 + (y^2)^2 - 2.(8x^2).y^2
= (8x^2 + y^2)^2 - 16.(xy)^2
= (8x^2 - 16xy + y^2)(8x^2 + 16xy + y^2)