【（二倍角的三角函数）cosπ/5cos2/5π的值是______cosπ/5cos2π/5=sinπ/5cosπ/5cos2π/5/sinπ/5=4sinπ/5cosπ/5cos2π/5/4sinπ/5=2sin2π/5cos2π/5/4sinπ/5=sin4π/5/4sinπ/5=sin(π-π/5)/4sinπ/5=(1/4)sinπ/5/sinπ/5=1/4但从4sinπ】

　　（二倍角的三角函数）

　　cosπ/5cos2/5π的值是______

　　cosπ/5cos2π/5

　　=sinπ/5cosπ/5cos2π/5/sinπ/5

　　=4sinπ/5cosπ/5cos2π/5/4sinπ/5

　　=2sin2π/5cos2π/5/4sinπ/5

　　=sin4π/5/4sinπ/5

　　=sin(π-π/5)/4sinπ/5

　　=(1/4)sinπ/5/sinπ/5

　　=1/4

　　但从4sinπ/5cosπ/5cos2π/5/4sinπ/5

　　=2sin2π/5cos2π/5/4sinπ/5

　　=sin4π/5/4sinπ/5

　　=sin(π-π/5)/4sinπ/5

　　=(1/4)sinπ/5/sinπ/5

　　=1/4开始没看懂!不知道是怎么将式子变形的!