x1+x2=-b/a
x1*x2=c/a
p=x1+x2=-b/a
q=x1^2+x2^2=(x1+x2)^2-2x1*x2=b^2/a^2-2c/a
r=x1^3+x2^3=(x1+x2)*[(x1+x2)^2-3x1*x2]=-b^3/a^3+3bc/a^2
ar+bq+cp=a*(-b^3/a^3+3bc/a^2)+b*(b^2/a^2-2c/a)-bc/a=0