P T Đ T T N T

LTKH

$x^7+x^5+x^3$

$=x^3\left(x^4+x^2+1\right)$

$=x^3\left\lbrace\left\lbrack\left(x^2\right)^2+2.x^2.1+1\right\rbrack-x^2\right\rbrace$

$=x^3\left\lbrack\left(x^2+1\right)^2-x^2\right\rbrack$

$=x^3\left(x^2+1+x\right)\left(x^2+1-x\right)$.