4. Chưa chắc cách làm nên làm thử `2` phương án:
`PH1:`
`x^4 + 1/4`
`=x^4+x^2-x^2+1/4`
`=(x^4+x^2+1/4)-x^2`
`=[(x^2)^2+2.x^2=(x^2+1/2)^2-x^2 . 1/2+(1/2)^2]-x^2`
`=(x^2+x+1/2)(x^2-x+1/2)`
`PH2:`
`x^4 + 1/4`
`= 1/4(4x^4+1)`
`=1/4(4x^4 - 4x^3 + 2x^2 + 4x^3 - 4x^2 + 2x + 2x^2 - 2x +1)`
`=1/4[2(2x^4 - 2x^3 + x^2) + 2(2x^3 - 2x^2 + x) + 2x^2 - 2x + 1]`
`=1/4[2(2x^4 - 2x^3 +x^2) + 2x(2x^2 - 2x + 1) + 2x^2 - 2x + 1]`
`=1/4[2(2x^4 - 2x^3 + 1x^2 ) + 2x(2x^2 - 2x +1) + 2x^2 - 2x + 1]`
`=1/4[2(2x^2 - 2x +1) x^2 + 2x(2x^2 - 2x + 1) + 2x^2 - 2x + 1]`
`=1/4(2x^2 - 2x +1)(2x^2 + 2x + 1)`
5.
`81x^4 + 4`
`=(9x^2)^2+36x^2+2^2-36x^2`
`=(9x^2)^2+36x^2+2^2-(6x)^2`
`=(9x^2+2)^2-(6x)^2`
`=(9x^2+2-6x)(9x^2+2+6x)`
9.
`x^4 + 64`
`=(x^4 + 16x^2 + 64) - 16x^2`
`=(x^2 + 8)^2 - (4x)^2`
`=(x^2 - 4x + 8)(x^2 + 4x + 8)`
10.
`81x^4 + 4y^4`
`=(9x^2)^2+2.9x^2 . 2y^2+(2y)^2-36x^2y^2`
`=(9x^2+2y^2)^2-(6xy)^2`
`=(9x^2+2y^2-6xy)(9x^2+2y^2+6xy)`
