If you don't build a bridge using the solution pattern, it takes a lot longer to win the game. The point of the tactis trainer is to make you see and choose the most forcing / fastest / strongest lines. I'm also annoyed when my mate-in-five gets 0 points because there was a mate-in-four on the board, but that's how it is :P
Reporting a flawed tactic problem
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I won the game very quickly after Rc6 against an expert (2000+) computer. Considering I'm only a mere beginner of 1350, it stands to reason that Rc6 is a perfectly correct alternate move.
I invite other to try it themselves. Go to this link http://www.chess.com/tactics/?id=188720&practice=1
And click "Try vs. Computer" and set it to expert rating, and see what happens when you play 2.Rc6. You'll find a quick easy victory.
Problem number 0188720 is flawed in my opinion.
It is white to play. Seen below are the sequence of moves that the tactic gives:
For white's second move, I played Rc6 and achieved an easy victory after trying playing against the computer on an expert (2000+) rating. Therefore Rc6 is a perfectly fine move and should not be considered incorrect.
http://www.chess.com/tactics/?id=188720