Chứng minh: (x + a) (x + b) = x^2 + (a + b)x + ab

chứng minh
$$\begin{gathered} a) \left(x+a\right)\left(x+b\right)=x^2+\left(a+b\right)x+ab \\ b) \left(x+a\right)\left(x+b\right)\left(x+c\right)=x^3+\left(a+b+c\right)x^2+\left(ab+bc+ca\right)x +abc \end{gathered}$$