how to partition a mounted ext4 with 445gb.im in trouble please help..when i try to unmount it shows you can't..

You cannot work on a partition that is mounted. And you cannot unmount the partitions that are in use by the current boot process.

So if you want to repartition or resize your main hard disk, you have to boot from a different medium. This can be an Ubuntu installation DVD or a bootable USB stick with an Ubuntu installation it on it (use the 'try Ubuntu without installing' mode).

how to boot from an iso image.i am new to ubuntu..could you please tell me the details..please
if i boot from iso then partition is possible ?

see https://help.ubuntu.com/community/GettingUbuntu

Read the first part "Downloading the Ubuntu ISO" how to download the .iso file (if you do not yet have one) and how to burn a DVD with it or how to create a bootable USB stick from it.

If you boot into an Ubuntu installation kit in the 'try Ubuntu without installing' mode, you should be able to access your hard disk in a mode that allows repartitioning.

One word of caution: Due to the fact that mistyping during the repartitioning activities can cause data loss, you should care for a backup of your data before starting.

If you are new to Ubuntu, why do you want to go to advanced techniques like repartitioning the hard disk?
What do you want to achieve, why does the current partition layout not suit your needs?

do i really repartition the hdd to installling windows 7(dual boot). is it possible to install it into the boot space of ubuntu(ext 4)
gparted shows like this
/dev/sda1 - fat32 dellutility 350mb
/dev/sda2 -fat32 os 3gb
dev/sda3 (encrypted) -ext4/ 454.61gb(boot)
dev/sda4 (encrypted) -extended 7.81gb
dev/sda5(encrypted) -linux swap 7.81gb
unallocated -unallocated 1.02mb

The current layout of the hard disk does not support an easy repartitioning to enable dual boot. And the encryption makes it even harder.

I recommend that you start reading https://help.ubuntu.com/community/HowtoPartition

then i have to do for dual booting windows along with ubuntu..

then what i have to do for dual booting windows along with ubutnu

For setting up your system as dual boot with Ubuntu and Windows you have to create several different partitions on your hard disk.
Since there are so many potential pitfalls and so many dependencies on the specifics of your system, I am not able to give an easy-to-follow recipe.

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