$A=9x^2-24xy+16y^2$
$=(3x)^2-2.3x.4y+(4y)^2$
$=(3x-4y)^2$
$B=4x^2-20xy+25y^2$
$=(2x)^2-2.2x.5y+(5y)^2$
$=(2x-5y)^2$
$C=x^4+6x^3+11x^2+6x+1$
$=(x^4+1)+6x.(x^2+1)+11x^2$
$=(x^2+1)^2+6x.(x^2+1)+11x^2-2x^2$
$=(x^2+1)^2+6x.(x^2+1)+9x^2$
$=(x^2+1)^2+2.(x^2+1).3x+(3x)^2$
$=(x^2+3x+1)^2$
$E=x^4-4x^3+2x^2+4x+1$
$=(x^4+1)-4x.(x^2-1)+2x^2$
$=(x^2-1)^2-4x.(x^2-1)+2x^2+2x^2$
$=(x^2-1)^2-2.(x^2-1).2x+(2x)^2$
$=(x^2-2x-1)^2$
Chúc bạn học tốt !