Lời giải:

a. $A=9x^2+15x+6xy+y^2+5y=(9x^2+6xy+y^2)+(15x+5y)$
$=(3x+y)^2+5(3x+y)=0^2+5.0=0$

b. $25x^2-y^4-5x+y^2=(25x^2-y^4)-(5x-y^2)=(5x-y^2)(5x+y^2)-(5x-y^2)$

$=(5x-y^2)(5x+y^2-1)$

9x²-y²+10ym-25m²=(3x)²-y²+2.5.ym-(5m)²

= -(y²-2.5.ym+(5m)²+(3x)²

=-(y-5m)²+(3x)²

=(3x)²-(y-5m)²

=(3x-y+5m)(3x+y+5m)

a) x²-4y²-x++2y

= x²-(2y)²-x+2y

= (x-2y)(x+2y)-(x-2y)

=(x-2y)(x+2y-1)

a) \(=x^2+7x-12x-84-2x+14\)

\(=x^2-7x-70\)

b)\(=x^2-4x-2x+8\)

\(=x\left(x-4\right)-2\left(x-4\right)\)

 \(=\left(x-4\right)\left(x-2\right)\)

c) \(=9x\left(x+y\right)-\left(x+y\right)\)

\(=\left(9x-1\right)\left(x+y\right)\)

d)\(=\left(x-y\right)^2-9^2\)

\(=\left(x-y+9\right)\left(x-y-9\right)\)

e)\(=x^2+8x+16-60+15x\)

\(=x^2+23x-44\)

a) Ta có: \(\left(x-12\right)\left(x+7\right)-2x-14\)

\(=\left(x-12\right)\left(x+7\right)-2\left(x+7\right)\)

\(=\left(x+7\right)\left(x-14\right)\)

b) Ta có: \(x^2-6x+8\)

\(=x^2-2x-4x+8\)

\(=\left(x-2\right)\left(x-4\right)\)

c) Ta có: \(9x^2+9xy-\left(x+y\right)\)

\(=9x\left(x+y\right)-\left(x+y\right)\)

\(=\left(x+y\right)\left(9x-1\right)\)

d) Ta có: \(\left(x^2-2xy+y^2\right)-81\)

\(=\left(x-y\right)^2-81\)

\(=\left(x-y-9\right)\left(x-y+9\right)\)

\(A=\left(6x-3y\right)+\left(4x^2-4xy+y^2\right)=3\left(2x-y\right)+\left(2x-y\right)^2=\left(2x-y\right)\left(2+2x-y\right)\)

\(B=9x^2-\left(y^2-4y+4\right)=9x^2-\left(y-2\right)^2=\left(3x-y+2\right)\left(3x+y-2\right)\)

\(C=-25x^2+y^2-6y+9=\left(y^2-6y+9\right)-25x^2=\left(y-3\right)^2-\left(5x\right)^2=\left(y-3-5x\right)\left(y-3+5x\right)\)\(D=x^2-4x-y^2-8y-12=\left(x^2-4x+4\right)-\left(y^2+8y+16\right)=\left(x-2\right)^2-\left(y+4\right)^2=\left(x-2-y-4\right)\left(x-2+y+4\right)=\left(x-y-6\right)\left(x+y+2\right)\)

a: Ta có: \(A=\left(6x-3y\right)+\left(4x^2-4xy+y^2\right)\)

\(=3\left(2x-y\right)+\left(2x-y\right)^2\)

\(=\left(2x-y\right)\left(2x-y+3\right)\)

b: Ta có: \(B=9x^2-\left(y^2-4y+4\right)\)

\(=9x^2-\left(y-2\right)^2\)

\(=\left(3x-y+2\right)\left(3x+y-2\right)\)

\(a,=\left(3x+1\right)^2-y^2=\left(3x-y+1\right)\left(3x+y+1\right)\\ b,=x\left(x^2-5x+6\right)=x\left(x^2-2x-3x+6\right)=x\left(x-2\right)\left(x-3\right)\)

a: \(81x^5-x^3\)

\(=x^3\left(81x^2-1\right)\)

\(=x^3\left(9x-1\right)\left(9x+1\right)\)

b: \(9x^2y-12xy+4y\)

\(=y\left(9x^2-12x+4\right)\)

\(=y\left(3x-2\right)^2\)

c: \(\left(5-x\right)^2-16\left(x-2\right)^2\)

\(=\left(x-5\right)^2-\left(4x-8\right)^2\)

\(=\left(x-5-4x+8\right)\left(x-5+4x-8\right)\)

\(=-3\left(x-1\right)\left(5x-13\right)\)

d: Ta có: \(9x^2-y^2-21x-7y\)

\(=\left(3x-y\right)\left(3x+y\right)-7\left(3x+y\right)\)

\(=\left(3x+y\right)\left(3x-y-7\right)\)

e: Ta có: \(-y^2+8y-16+9x^2\)

\(=-\left(y^2-8y+16-9x^2\right)\)

\(=-\left(y-4-3x\right)\left(y-4+3x\right)\)

f: Ta có: \(5x^2-4x-1\)

\(=5x^2-5x+x-1\)

\(=5x\left(x-1\right)+\left(x-1\right)\)

\(=\left(x-1\right)\left(5x+1\right)\)

\(=12x^2-6x=6x\left(2x-1\right)\)

Ta có:  9 x 2 - 1 = ( 3 x ) 2 - 1 2 = ( 3 x - 1 ) ( 3 x + 1 )