>> a=-8;>> r_a=a^(1/3)
>> p=[1,0,0,-a];
>> R=roots(p)
>> MR=abs(R(1));
>> t=0:pi/20:2*pi;
>> x=MR*sin(t);
>> y=MR*cos(t);
>> plot(x,y,'b:'),grid on
>> hold on
>> plot(R(2),'.','MarkerSize',30,'Color','r')
>> plot(R([1,3]),'o','MarkerSize',15,'Color','b')
>> axis([-3,3,-3,3]),axis square
>> hold off
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