but as i've written above can't do this photo by photo, as with crop solution; imagine to have even 1000 photos, is not possible to me
and i want to have in the library the whole photo, due to the fact i have to export in several ways (video display, printing, web), so cropping is definitly not the way to me (but, i repeat, thank you for the answer)
Any other suggestion even with plugins?
Cropping is not destructive, so even if you crop a picture, the "whole photo" is still there in the library. You can always go back to it and undo or change the crop.
You can crop a batch of pictures using the Sync... feature in LR. Select the pictures, go to the Develop module, crop the first picture to the desried aspect ratio (16:9 or 16:10 as suggested above), then click on Sync... Clear all the checkmarks, then click on the Aspect Ratio box under Crop and click on Synchronize.