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how to show blog comments in feed manager ?
I don't think there is anything built in for that. If using Disqus or IntenseDebate, they may have an rss feed for comments, I'm not sure.
then what is the Add link of comments to the RSS (or similar, don't remember ..) in the feed settings of the blog ?
I think that just adds a link at the end of a post in the feed that links back to the original post so that people who want to comment or view comments can go to the original post to do that.