Bài 3:
a)\(x^3+2x+x^2\\ =x\left(x^2+2+x\right)\)
b)\(x^2+2xy-9+y^2\\ =\left(x^2+2xy+y^2\right)-3^2\\ =\left(x+y\right)^2-3^2\\ =\left(x+y+3\right)\left(x+y-3\right)\)
c)\(x^2-3xy-10y^2\\ =x^2-5xy+2xy-10y^2\\ =\left(x^2-5xy\right)+\left(2xy-10y^2\right)\\ =x\left(x-5y\right)+2y\left(x-5y\right)\\ =\left(x-5y\right)\left(x+2y\right)\)