Specific not work "start" FILE FOR VIDEO

[codiac82]

I have 70 gigabytes of video and I'll extract a fragment of a size of 10 megabytes
Example:

1. fs.createReadStream('sample.txt', {start: 50 000 000, end: 60 000 000})
   2.res.writeHead(206, {
   'Content-disposition': 'filename=streamxx.mp4',
   'Accept-Ranges': 'bytes',
   'Content-Type': "video/mp4",
   'Content-Range': 'bytes ' + start + '-' + end + '/' + GridFile.length,//GridFile.length, // start = 50 000 000
   'Content-Length': chunksize
   });
2. GridFile.seek(start, function () { // start = 50 000 000
   or Other

These examples can extract part of the video file, but can not play in this part of the HTML5 Video tag . I tried FFMPEG and its superstructure but still can not play. As this fragmetn play or what or how to modify it ??