10: Ta có: \(\left(2x+3\right)^2-\left(x+1\right)^2\)
\(=\left(2x+3-x-1\right)\left(2x+3+x+1\right)\)
\(=\left(x+2\right)\left(3x+4\right)\)
9: Ta có: \(\left(x+y\right)^2+\left(x-y\right)^2\)
\(=x^2+2xy+y^2+x^2-2xy+y^2\)
\(=2\left(x^2+y^2\right)\)
\(1,\dfrac{x^2}{4}-2xy+4y^2\)
\(2,=2x^2-2\sqrt{2}.xy+y^2\)
3,\(\dfrac{9}{4}x^2+9xy+9y^2\)
4, \(2x^2+8xy+8y^2\)
5,\(x^2+\dfrac{1}{3}xy+\dfrac{1}{36}y^2+y+9+6x\)