\(b.9a^2+b^2+6ab\\ =\left(3a\right)^2+2\cdot\left(3a\right)\cdot b+b^2\\ =\left(3a+b\right)^2\\ c.25x^2+4y^2-20xy\\ =\left(5x\right)^2-2\cdot5x\cdot2y+\left(2y\right)^2\\ =\left(5x-2y\right)^2\\ x^2-x+\dfrac{1}{4}\\ =x^2-2\cdot x\cdot\dfrac{1}{2}+\left(\dfrac{1}{2}\right)^2\\ =\left(x-\dfrac{1}{2}\right)^2\)