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I KNOW he hit a third five.

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No you don't. I've seen a lot of players whom I had designated "loose-passive" who will bluff-raise on the river when a scare card comes. "loose-passive" is a broad category, and doesn't always accurately indicate whether the player is capable of making a move like this.

The standard question is this: is it worth risking one big bet to win 17? In other words, are you more than 94% sure that you're beaten? Not without a strong read. Huge pot; don't dump it, and don't punch yourself either.