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設cos15°-sin15°=x, (cos15°-sin15°)²;=x²;, cos²;15°-2sin15°cos15°+sin²;15°=x²;, 1-sin30°=x²; 1-1/2=x²;, ∴x²;=1/2 x=√2/2(∵cos15°-sin15°>0,∴x=-√2/2舍去).
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