代入x = 0可得f(0) = 0.代入x = π/k,可得f(π/k) = -cos(π/k)^k-cos(2π/k)^k.若f(x)不依赖于x,则有-cos(π/k)^k-cos(2π/k)^k = f(π/k) = f(0) = 0.若k为偶数,有-cos(π/k)^k ≤ 0且-cos(2π/k)^k ≤ 0.而由cos(...