注意到 cos(π-x)=-cos(x) 所以左端首尾相加是0,所以2(cos(π/2k+1)+cos(2π/2k+1)+…+cos(2k-1)π/2k+1+cos2kπ/2k+1)=(cos(π/2k+1)+cos2kπ/2k+1)+(cos(2π/2k+1)+cos(2k-1)π/2k+1)...+(cos2kπ/2k+1+cos(π/2k+...