fredthebear: <WhiteRook48: was it 42. Rxf5?>
Yes. 42.Rxf5 eventually wins fairly easily. White will have a space advantage and three connected pawns on the kingside (the f-pawn is already passed and either of the other two can become passed by advancing the g-pawn at the proper time).
Exchanging rooks 42...RxRf5 43.gxRf5 would make matters even worse for Black. Without rooks, the Black king would be forced to race across the board to stop the resulting passed White doubled f-pawns. Meanwhile, the White king has an easy time gobbling up the two nearby Black pawns on the queenside and then promoting his own b-pawn into a new queen to deliver checkmate.
Technique-wise after 42...RxRf5 43.gxRf5, White would at most need to move one pawn to f6 to trigger the Black king's mad dash across to stop it in time. Then the doubled White f-pawns sit and wait to be taken (buying time for their own king to mop up on the queenside). The trailing f-pawn definitely does not move; that would be an unnessary waste of time. Yet the Black king must still come and eliminate it too. Passed pawns dictate to the defender!
Of course, if the Black king foolishly stays on the queenside to protect his pawns there (or returns to the queenside), White obviously would go ahead and promote the f-pawn. In other words, after the rooks are exchanged, it's much quicker (fewer moves) for the White king to do his job in the resulting king and pawn ending. The Black king cannot be in two places at once -- queenside and kingside.
Furthermore, the Black pawns are nearly out of safe moves and make little contribution. The White b-pawn will not hesitate to capture the Black a-pawn should it try to sneak past. This outward pawn capture would make the b-pawn a devastating outside passer on the a-file, so Black definitely should not try pushing the a-pawn up the board.
Black would be better off not exchanging rooks in principle, but is still eventually lost after 42.Rxf5. Remember the rule of thumb: When down material, exchange pawns not pieces (the concept of which created the lost position?! -- Black is just giving away his f-pawn in the game's final move instead of truly exchanging it off for another pawn). By keeping his last rook on the board, Black can delay defeat longer by having more moves available in the resistance. If the 4-3 pawn ratio could be reduced to 1-0 and the rooks still aboard, Black is far more likely to draw. Black must remove those White pawns!
However, reducing the pawn ratio is not very likely to happen with White having three pawns safely connected on the other side of the Black king. These mobile pawns can push up the board toward promotion. Remember the rule of thumb: A lone rook cannot stop two adjacent passed pawns on the 6th rank from one promoting. What's more, White's rook could assist in the upcoming pawn roller. Thus, Black's resignation is definitely in order after he gives away his own f-pawn.