7.原式趋于[1/(2n)][1+cos(2π/n)+1+cos(4π/n)+……+1+cos((2n-2)π/n)],cos(2π/n)+cos(4π/n)+……+cos((2n-2)π/n)=[1/(2sin(π/n))][sin((2n-1)π/n)-sin(π/n）]=-1,所以原式趋于2(n-2)π/n趋于2π。
