老师能不能给我讲讲第14和15两道题呢,求求老师啦,感谢不尽
来源:学生作业帮 编辑:搜搜考试网作业帮 分类:数学作业 时间:2024/08/04 00:06:55
![](http://img.wesiedu.com/upload/5/c4/5c4e4186fc726baf9b6959f4c8f752b2.jpg)
![老师能不能给我讲讲第14和15两道题呢,求求老师啦,感谢不尽](/uploads/image/z/18989779-67-9.jpg?t=%E8%80%81%E5%B8%88%E8%83%BD%E4%B8%8D%E8%83%BD%E7%BB%99%E6%88%91%E8%AE%B2%E8%AE%B2%E7%AC%AC14%E5%92%8C15%E4%B8%A4%E9%81%93%E9%A2%98%E5%91%A2%EF%BC%8C%E6%B1%82%E6%B1%82%E8%80%81%E5%B8%88%E5%95%A6%EF%BC%8C%E6%84%9F%E8%B0%A2%E4%B8%8D%E5%B0%BD)
解题思路: 由正弦定理及b²=ac可得: sin²B=sinAsinC ∴cos(A-C)+cosB+cos2B =cos(A-C)-cos(A+C)+cos2B =2sinAsinC+cos2B =2sin²B+(1-2sin²B) =1.
解题过程:
答疑网规定教师一次回答一题,不好意思
15
由正弦定理及b²=ac可得:
sin²B=sinAsinC
∴cos(A-C)+cosB+cos2B
=cos(A-C)-cos(A+C)+cos2B
=2sinAsinC+cos2B
=2sin²B+(1-2sin²B)
=1.
解题过程:
答疑网规定教师一次回答一题,不好意思
15
由正弦定理及b²=ac可得:
sin²B=sinAsinC
∴cos(A-C)+cosB+cos2B
=cos(A-C)-cos(A+C)+cos2B
=2sinAsinC+cos2B
=2sin²B+(1-2sin²B)
=1.