[" 15.If "omega" is complex cube root of unity "],[" and "a,b,c" are three real numbers "],[" such that "],[(1)/(a+omega)+(1)/(b+omega)+(1)/(c+omega)=2 omega^(2)],[" and "],[(1)/(a+omega^(2))+(1)/(b+omega^(2))+(1)/(c+omega^(2))=2 omega],[" then "(1)/(a+1)+(1)/(b+1)+(1)/(c+1)" is "]