$18/_{}$ $x^2-2xy+y^2+3x-3y-10_{}$
$=x^2-xy-xy+y^2+5x-2x-5y+2y-10_{}$
$=x.(x-y-2)-y.(x-y-2)+5.(x-y-2)_{}$
$=(x-y-2)(x-y+5)_{}$
$19/_{}$ $x^4+4_{}$
$=x^4+4+4x^2-4x^2_{}$
$=(x^2+2)^2-4x^2_{}$
$=(x^2+2-2x)(x^2+2+2x)_{}$
$20/_{}$ $4x.(x+1)^2-5x^2.(x+1)-4.(x+1)_{}$
$=(x+1).[ 4x.(x+1)-5x^2-4]_{}$
$=(x+1).(4x^2+4x-5x^2-4)_{}$
$=(x+1)(-x^2+2x+2x-4)_{}$
$=(x+1).[ -x.(x-2)+2.(x-2)]_{}$
$=(x+1).[ -(x-2).(x-2)]_{}$
$=(x+1).[ -(x-2)^2]_{}$
$=-(x+1)(x-2)^2_{}$
$21/_{}$ $(1+2x)(1-2x)-(x+2)(x-2)_{}$
$=1-4x^2-(x^2-4)_{}$
$=1-4x^2-x^2+4_{}$
$=5-5x^2_{}$
$=5.(1-x^2)_{}$
$=5.(x+1)(x-1)_{}$
$22/_{}$ $a^2-2a-4b^2-4b_{}$
$=(a-2b)(a+2b)-2.(a+2b)_{}$
$=(a+2b)(a-2b-2)_{}$
