化简cosπ/5cos2π/5

COSπ/5乘以COS2π/5 求COSπ/5*COS2π/5 cosπ/5·cos2π/5等于多少? （cosπ/5）（cos2π/5)的值等于 cosπ/5 乘以cos2π/5等于多少 cosπ/5*cos2π/5 =(2sinπ/5*cosπ/5*cos2π/5)/(2sinπ/5) =(sin2π/5 （二倍角的三角函数）cosπ/5cos2/5π的值是______cosπ/5cos2π/5 =sinπ/5cosπ/5c cos派/5cos2派/5 已知sin(π/4-α)=5/13,α∈(0,π/4) 求cos2α/cos(π/4+α) cosπ/5乘以cos2π/5的值是多少?请给出具体解题过程,谢谢拜托各位大神 cosπ/5cos2π/5的值等于 ：麻烦过程竖着写 横着写看不懂~最好一步一步写清楚~ 已知sinα+cosα=1/5,且π/2≤α≤3π/4,求cos2α