急、COSπ/9*COS2π/9*COS4π/9=?同样,_数学解答

急、COSπ/9*COS2π/9*COS4π/9=?同样,

人气：204 ℃　时间：2020-04-09 09:55:15

解答

π/9=20°2π/9=40°4π/9=80°
cos20cos40cos80
=sin20cos20cos40cos80/sin20
=1/2*sin40cos40cos80/sin20
=1/4*sin80cos80/sin20
=1/8*sin160/sin20
=1/8*sin20/sin20
=1/8