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Alexey wrote '1*2*3*... 9 = 21' on the board, inserting a plus or minus signs in place of each little star. Sasha changed some of these signs to their opposites so that the result he got was 20. Prove that one of the boys made a mistake.​

User GolfBravo
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someone answer the question on page btw ion kn the answer to this
User Chunguiw
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