[tex3]a+b+c+d=0\implies\begin{cases}-a=b+c+d\\-b=a+c+d\\-c=a+b+d\\-d=a+b+c\end{cases}[/tex3]
[tex3]a+b+c+d=0\\(a+b+c+d)^3=0\\(a+b+c+d)(a^2+b^2+c^2+d^2+2ab+2ac+2ad+2bc+2bd+2cd)=0\\
a^3+b^3+c^3+d^3+6(abc+abd+acd+bcd)+3[a^2b+a^2c+a^2d+ab^2+b^2c+b^2d+ac^2+bc^2+c^2d+ad^2+bd^2+cd^2]=0\\a^3+b^3+c^3+d^3+6(abc+abd+acd+bcd)+3[a^2(b+c+d)+b^2(a+c+d)+c^2(a+b+d)+d^2(a+b+c)]=0\\
a^3+b^3+c^3+d^3+6(abc+abd+acd+bcd)+3[a^2(-a)+b^2(-b)+c^2(-c)+d^2(-d)]=0\\
a^3+b^3+c^3+d^3+6(abc+abd+acd+bcd)-3[a^3+b^3+c^3+d^3]=0\\-2(a^3+b^3+c^3+d^3)+6(abc+abd+acd+bcd)=0\\
3(abc+abd+acd+bcd)=a^3+b^3+c^3+d^3\\\frac{abc+abd+acd+bcd}{a^3+b^3+c^3+d^3}=\frac13[/tex3]
Espero ter ajudado
Saudações.