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So, here's the answer:
You ask either angel, "Which gate will the other angel tell me goes to hell?" Then, walk through the door they indicate. Here's why:
If you ask the lying angel, he will indicate the gate to heaven, because he knows the truthful angel will NOT indicate that gate, therefore, the lying angel will be lying whe indicating the gate.
If you ask the truthful angel, he'll know that the other angel will lie and indicate the gate to heaven because the lying angel always lies, so he will point to the gate to heaven as well.