\(a+b+c=0\Rightarrow\hept{\begin{cases}b+c=-a\\a+c=-b\\a+b=-c\end{cases}}\)
Ta có:
\(M=a\left(a+b\right)\left(a+c\right)=a.\left(-c\right).\left(-b\right)=abc\)(1)
\(N=b\left(b+c\right)\left(b+a\right)=b.\left(-a\right)\left(-c\right)=abc\)(2)
\(P=c\left(c+a\right)\left(c+b\right)=c.\left(-b\right).\left(-a\right)=abc\)(3)
Từ (1)(2)(3) suy ra M=N=P