Quod erat demonstrandum

From RationalWiki

Quod erat demonstrandum (normally abbreviated to Q.E.D.) is a Latin phrase that means literally "that which was to be demonstrated." In normal English, this means "thus it is proven." It is used formally after the last line of a proof, and colloquially in discussion or argument in a similar fashion.

<snark>
Poor example of usage:

• 10% of car thefts are committed by left-handed people.
• Half of all polar bears are left handed.
• There's a 5% chance your car was stolen by a polar bear.

Q.E.D.
</snark>

Never trust anyone who actually says "Q.E.D.", as they've probably just committed a logical fallacy, such as the above.