7. Let $\omega$ be a complex cube root of unity, then $\left(a+b \omega+c \omega^{2}\right)\left(a+b \omega^{2}+c \omega\right)=$
$\begin{array}{l}\text { 7. } a^{2}+b^{2}+c^{2}-a b-b c-c a \\ \left(a+b \omega+c \omega^{2}\right)\left(a+b \omega^{2}+c \omega\right)=a^{2}+b^{2}+c^{2}+a b \omega^{2}+a c \omega+a b \omega+b c \omega^{2}+c a \omega^{2}+b c \omega \\ =a^{2}+b^{2}+c^{2}-a b-b c-a c\end{array}$