>>30
>Σsinkx=(cos(1/2)x-cos(n+1/2)x)/2sin(x/2)
f(2x)=(cosx-cos(2n+1)x)/2sinx
2f'(2x)=(((2n+1)sin(2n+1)x-sinx)sinx-(cosx-cos(2n+1)x)cosx)/2sin^2x=((2n+1)sin(2n+1)xsinx+cos(2n+1)xcosx-1)/2sin^2x=((2n+1)(cos2nx-cos2(n+1)x)+cos2(n+1)x+cos2nx-1)/4sin^2x=(2(n+1)cos2nx-2ncos2(n+1)x-1)/4sin^2x