I know vous is used for plural but also for being polite. I also know that the adjective should agree with the subject. So, if I'm saying to two or more people that they're sympathetic, then I would write:
Vous êtes sympathiques!
And then sympatiques is written with an extra s
because I'm talking to more persons. But if I'm talking to one person, and use vous to be polite, is the sentence below correct?
Vous êtes sympathiques!
In this case, sympathiques agrees with the subject, but sympathiques is plural, and I'm talking to one person.
So, should I use sympathiques or sympathique when I'm talking to one person but use vous to be polite?