高一三角函数题,高人帮忙!
来源:百度知道 编辑:UC知道 时间:2024/06/17 16:54:17
1. cos36`-cos72`
2. (sin20`)^2+(sin80)^2+3^(1/2)*sin20`sin80`
要过程,越详细越好!!!急!!!
2. (sin20`)^2+(sin80)^2+3^(1/2)*sin20`sin80`
要过程,越详细越好!!!急!!!
cos36-cos72
=cos(54-18)-cos(54+18)
=cos54cos18+sin54sin18-[cos54cos18-sin54sin18]
=2sin54sin18
=-2sin54sin(-18)
=2sin54sin18
=2cos36sin18
=2cos36sin18cos18/cos18
=cos36sin36/cos18
=sin72/(2cos18)
=sin72/(2sin72)=1/2
(一)利用黄金三角形及余弦定理,可求得:cos36=[1+√5]/4,cos72=[-1+√5]/4.故原式=1/2.(二)