\(x^2-y^2+10x+8y+9\)
\(=x^2-y^2+9x+x+9y-y+9\)
\(=\left(x^2-y^2\right)+\left(x-y\right)+\left(9x+9y+9\right)\)
\(=\left(x-y\right)\left(x+y\right)+\left(x-y\right)+9\left(x+y+1\right)\)
\(=\left(x-y\right)\left(x+y+1\right)+9\left(x+y+1\right)\)
\(=\left(x+y+1\right)\left(x-y+9\right)\)
