Kent wrote:so r3c7=7->r3c9=9->r2c9=4->r1c8=8->r1c4=9->r3c6=8 and therefore the cell r3c7=7.Is that how it works?? So since u start from r3c7=7 and at the end u also get r3c7=7, so that mean it's correct right?

No. If you get a contradition, you know your assuption was wrong. If you don't, your assuption may or may not be wrong -- unless of course you take it all the way to the end and solve the puzzle.

The statement ALL CROWS ARE BLACK is disproved by a single WHITE crows, but 1000 black crows proves nothing. The white crow is proof by contradiction.

If all possibilities lead to the same conclusion, that conclusion must be true. That's a forcing chain.

If an assumption leads to a contradiction, that assumtion is false. That's proof by contradiction.