Given that,
#abc=1," we have, "1/c=ab, and, c=1/(ab)......(star)#.
#:. sum1/(1+a+1/b)#,
#=1/(1+a+1/b)+1/(1+b+1/c)+1/(1+c+1/a)#,
#=1/(1+a+1/b)+1/(1+b+ab)+1/(1+1/(ab)+1/a)......[because, (star)]#,
#=b/{b(1+a+1/b)}+1/(1+b+ab)+(ab)/{ab(1+1/(ab)+1/a)}#,
#=b/(b+ab+1)+1/(1+b+ab)+(ab)/(ab+1+b)#,
#=(b+1+ab)/(b+1+ab)#,
#=1#.