f'(x)=x'*(x-1)(x-2)..........(x-1000) +x*[(x-1)(x-2)..........(x-1000)]';
f'(0)=1*(0-1)(0-2).........(0-1000)+0*[(x-1)(x-2)..........(x-1000)]'=(-1)*(-2).......(-1000)=1000!
f'(x)=x'*(x-1)(x-2)..........(x-1000) +x*[(x-1)(x-2)..........(x-1000)]';
f'(0)=1*(0-1)(0-2).........(0-1000)+0*[(x-1)(x-2)..........(x-1000)]'=(-1)*(-2).......(-1000)=1000!