As an alternative to Juan's answer (roughly in the same spirit), I suggest that you can phrase the email regarding the issue itself--since presumably the professor would also be interested in knowing the answer to the problem. Something like the following sound suffice.

Hi Professor XY, recently I learnt about [...], which seemed really interesting to me because [...]. If you recall, this is similar to what we discussed a while ago regarding ABC, which is what prompted me to look into this further. I thought I would send this to you in case you happened to be interested in it. Have a nice day!

The point is to focus on the part which the professor would also be curious about/interested in, and to not dwell on the fact that they weren't aware of the answer beforehand.

As an alternative to Juan's answer (roughly in the same spirit), I suggest that you can phrase the email regarding the issue itself--since presumably the professor would also be interested in knowing the answer to the problem. Something like the following should suffice.

Hi Professor XY, recently I learnt about [...], which seemed really interesting to me because [...]. If you recall, this is similar to what we discussed a while ago regarding ABC, which is what prompted me to look into this further. I thought I would send this to you in case you happened to be interested in it. Have a nice day!

The point is to focus on the part which the professor would also be curious about/interested in, and to not dwell on the fact that they weren't aware of the answer beforehand.