sin(270°+15°)请用两角差的余弦公式求解

问题描述:

sin(270°+15°)请用两角差的余弦公式求解
答案给的是=-cos15°=-cos(60°-45 °)

sin(270°+15°)=sin270°cos15°+cos270°sin15°
=-1*cos15°+0*sin15°
=-cos15°
=-cos(45°-30°)
=-(cos45°cos30°+sin45°sin30°)
=-[(√2/2)*(√3/2)+(√2/2)*(1/2)]
=-(√6+√2)/4