\(a,x^2+2x+1=x^2+2.x.1+1^2=\left(x+1\right)^2\)
\(b,9x^2+y^2+6xy=9x^2+6xy+y^2=\left(3x\right)^2+2.3.x.y+y^2=\left(3x+y\right)^2\)
\(c,25a^2+4b^2-20ab\)
\(=25a^2-20ab+4b^2\)
\(=\left(5a\right)^2-2.5.2.a.b+\left(2b\right)^2\)
\(=\left(5a-2b\right)^2\)
\(d,x^2-x+\dfrac{1}{4}=x^2-2.x.\dfrac{1}{2}+\left(\dfrac{1}{2}\right)^2=\left(x-\dfrac{1}{2}\right)^2\)