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∵f(x)=-x3+ax,∴f′(x)=a-3x2,∵函數f(x)=ax-x3在區間[1,+∞)上單調遞減,∴f′(x)=a-3x2≤0在區間[1,+∞)上恒成立,∴a≤3x2在區間[1,+∞)上恒成立,∴a≤3.故選C.
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