cos^15°-sin^15°=

Let cos15 ° - sin15 ° = X,
（cos15°-sin15°）²=x²,
cos²15°-2sin15°cos15°+sin²15°=x²,
1-sin30°=x²
1-1/2=x²,
∴x²=1/2
X = √ 2 / 2 (∵ cos 15 ° - Sin 15 ° > 0, ∵ x = - √ 2 / 2, rounded off)

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