laptop: intel integrated AND Nvidia GeForce GT 840M

Nope, just don't forget to add the acpi_osi= parameter to GRUB either with

sudo dpkg-reconfigure grub-pc

or by manually editing the /etc/default/grub file, which prevents the use of both GPUs at the same time (and should save you battery power). You will be then able to switch the default driver using the

sudo update-alternatives --config x86_64-linux-gnu_gl_conf

command, in case you'll choose to use the proprietary NVIDIA driver later.

This is just for save battery, right?

I can use my laptop without freedom problem?

Thanks you.

No, it should also disable the problematic Nouveau driver, that may cause issues. Just try for yourself.

Thanks.
Just for understanding..

As I will use only Free and Open Source Software, I have two questions

1) I'will have problem regarding non-free drivers or binary blob?

2) Will FOOS prevent me from loading or using binary data loaded in BIOS or in the CARD ?

Thanks for you patience

The BIOS is out of the scope of the operating system. Trisquel, more precisely its kernel (Linux-libre), does not contain any blob. I do not know about you specific model but nVidia cards are not as bas as AMD's: it should work fine and you will probably even get some 3D acceleration. Try the Live system (no need to install right away) and see by yourself.

Anyway, stick to Intel graphical chipsets for your next purchases. Linux-libre perfectly handles them and you will spend less money and less energy.