a) \(x^2+3xy+x^2y=x\left(x+3y+xy\right)\)
b) \(\left(a+b\right)^3+\left(a+b\right)^2=\left(a+b\right)^2\left(a+b+1\right)\)
c) \(x^4-y^4+x^2+y^2=\left(x^2-y^2\right)\left(x^2+y^2\right)+\left(x^2+y^2\right)=\left(x^2+y^2\right)\left(x^2-y^2+1\right)\)