Question

计算:cos15°-sin15°.

Answer 1

解法一:原式=cos(45°-30°)-cos(45°+30°)

=cos45°cos30°+sin45°sin30°-cos45°cos30°+sin45°sin30°

=2sin45°·sin30°=2× × = .

解法二:原式= ( cos15°- sin15°)

= (cos45°cos15°-sin45°sin15°)

= cos(45°+15°)= cos60°= .