首先,余弦函数是偶函数,即cos(-3π/5)=cos(3π/5)
然后,由于3π/5是钝角,又根据余弦函数图像得,cos(3π/5)=-cos(3π/5-π/2)=-cos（π/10)
综上,cos(-3π/5))=-cos（π/10)