Question

cos^15°-sin^15°=

Answer 1

设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（∵cos15°-sin15°＞0,∴x=-√2/2舍去）.