How should I tell a professor the answer to something he doesn't know?
I think there is an easy way to phrase this to make it tactful, after all professors are often curious about learning new things too! 


  "Hello Professor X, 
  I hope you've been well! Last year I asked you a question and we couldn't figure it out at the time but I've since come across an interesting answer and just wanted to pass along the info just in case you're curious. We were discussing Y ... "