Formule pt. calcul prescurtat

$$\begin{gathered} {\left(a+b\right)}^2=a^2+2ab+b^2 \\ {\left(a-b\right)}^2=a^2-2ab+b^2 \\ \left(a+b\right)\left(a-b\right)=a^2-b^2 \\ {\left(a+b+c\right)}^2=a^2+b^2+c^2+2ab+2ac+2bc. \end{gathered}$$