\(a,\left(x+y\right)^3-x^3-y^3=\left(x+y\right)^3-\left(x^3+y^3\right)\)
\(=\left(x+y\right)^3-\left(x+y\right)\left(x^2-xy+y^2\right)\)
\(=\left(x+y\right)\left[\left(x+y\right)^2+x^2-xy+y^2\right]\)
\(=\left(x+y\right)\left(x^2+2xy+y^2+x^2-xy+y^2\right)\)
\(=\left(x+y\right)\left(2x^2+xy+2y^2\right)\)
\(b,x^2+6xy+9y^2=\left(x+3y\right)^2\)
=.=......hok tốt nha!!!