11. $x^3+3x^2+3x+1$
$=(x+1)^3$
12. $x^3-3x^2+3x-1$
$=(x-1)^3$
22. $125-x^6$
$=5^3-(x^2)^3$
$=(5-x^2)(25+5x^2+x^4)$
20. $49(y-4)^2-9(y+2)^2$
$=[7(y-4)]^2-[3(y+2)]^2$
$=[(7(y-4)+3(y+2)][(7(y-4)-3(y+2)]$
$=(7y-28+3y+6)(7y-28-3y-6)$
$=(10y-22)(4y-34)$
$=4(5y-11)(2y-17)$
15. $-25x^6-y^8+10x^3y^4$
$=-[(5x^3)^2-2.5x^3.y^4+(y^4)^2]$
$=-(5x^3-y^4)^2$.
