\(a,=\left(x-3\right)\left(x+3\right)\\ b,=\left(2x-5\right)\left(2x+5\right)\\ c,=\left(x^3-y^3\right)\left(x^3+y^3\right)\\ =\left(x-y\right)\left(x+y\right)\left(x^2-x+1\right)\left(x^2+x+1\right)\\ d,=\left(3x+y\right)^2\\ e,=-\left(x-3\right)^2\\ f,=\left(x+2y\right)^2\)
\(a.\\ x^2-9=\left(x-3\right)\left(x+3\right)\\ b.\\ 4x^2-25=\left(2x\right)^2-5^2\\ =\left(2x-5\right)\left(2x+5\right)\\ c.\\ x^6-y^6=\left(x^2\right)^3-\left(y^2\right)^3\\ =\left(x^2-y^2\right)\left(x^4+x^2y^2+y^4\right)\\ =\left(x-y\right)\left(x+y\right)\left(x^2+y^2+xy\right)\left(x^2+y^2-xy\right)\\ \)
\(d.\\ 9x^2+6xy+y^2=\left(3x\right)^2+2\cdot3\cdot x\cdot y+y^2\\ =\left(3x+y\right)^2\\ e.\\ 6x-9-x^2=-\left(x^2-6x+9\right)\\=-\left(x-3\right)^2\\ f.\\ x^2+4y^2+4xy=x^2+4xy+\left(2y\right)^2=\left(x+2y\right)^2\)
