三角函数证明，急！！！

利用恒等式
n
∑ cos（2k-1）π/(2n+1)=1/2
K=1
得x+y
= cos(2π/2n+1)+cos(4π/+2n+1)+ ...+cos(2nπ/2n+1)=1/2

{恒等式证明如下：（当然可以略过）
设π/(2n+1)=θ
n n
sinθ* ∑cos(2k-1)θ = ∑(sinθ*cos(2k-1)θ)
k=1 k=1
n
=1/2 ∑(sin2kθ-sin(2k-2)θ)
K=1
n n
=1/2 (∑sin2kθ- ∑sin(2k-2)θ)=1/2 sin2nθ ...........（将θ代入）
k=1 k=1
=1/2 sin2nπ/(2n+1)=1/2sin(π-π/(2n+1))

=1/2sinπ/(2n+1)=1/2 sinθ
n
即 ∑ cos（2k-1）π/(2n+1)=1/2 }
K=1