$1)x^2+2x+1=(x+1)^2$
$2)9x^2+y^2+6xy= (3x)^2+2.3x.y +y^2 = (3x+y)^2$
$3)25a^2+4b^2-20ab=(5a)^2 -2.5a.2b +(2b)^2=(5a-2b)^2$
$4)x^2-x+\dfrac{1}{4}=x^2 -2.x.\dfrac{1}{2}+ (\dfrac{1}{2})^2 = (x-\dfrac{1}{2})^2$
$5)9x^2+6x+1=(3x)^2 + 2.3x.1 + 1^2 = (3x+1)^2$
$6) 49x^2-70x+25 tại x=5$
$=> (7x)^2 - 2.7x.5 + 5^2= (7x-5)^2$
$x= 5 => ( 7.5-5)^2 = 30^2 = 900$