Giải thích các bước giải :
`a)x^4+4=x^4+4x^2+4-4x^2=(x^2+2)^2-(2x)^2=(x^2-2x+2)(x^2+2x+2)`
`b)x^3-2x-4`
`=(x^3-2x^2)+(2x^2-4x)+(2x-4)`
`=x^2(x-2)+2x(x-2)+2(x-2)`
`=(x-2)(x^2+2x+2)`
`c)2x^3-7x^2-13x+3`
`=(2x^3+3x^2)-(10x^2+15x)+(2x+3)`
`=x^2(2x+3)-5x(2x+3)+(2x+3)`
`=(2x+3)(x^2-5x+1)`
`d)3x^5-5x^4-2x^3+6x^2+5x+1`
`=(3x^5+x^4)-(6x^4+2x^3)+(6x^2+2x)+(3x+1)`
`=x^4(3x+1)-2x^3(3x+1)+2x(3x+1)+(3x+1)`
`=(3x+1)(x^4-2x^3+2x+1`
