求cos165°的值

求cos165°的值.
2025-06-22 22:08:26
推荐回答(1个)
回答1:

cos165°
=cos(180°-15°)
=-cos15°
=-cos(45°-30°)
=-(cos45°cos30°+sin45°sin30°)
= -(
2
2
?
3
2
+
2
2
?
1
2
)=-
6
+
2
4
.