Hmmm. Prejudice - Pre-judge: I take that to mean making assumptions about a person based on their proximity to some group (IE: ethnicity, religion, etc) without actually knowing anything about that person.
I don't think your 1+1=3 example counts as prejudice...there's no way you could know that particular thought/belief without knowing that individual to some extent. As you say, it comes down to personal choice...but personal is down to the individual. To be clear, I think it's totally fine to write off an individual based on their individual beliefs. Where it gets wrong to me is to write-off an entire segment of humanity based on some assumption of shared identity when individuals in that segment are all...individual. For instance, most Iranians are nominally Muslim...which means that some people would make broad assumptions about all of them. What they fail to account for is that for many among them, this isn't really a choice.
Basically, get to know someone before juding them. Don't pre-judge, post-judge! Being postjudice is something we should all strive for.