Sure yu could do it, but why would you want to?
First off, the speed. A traditional computer is so much faster. Sure the iPad is fast for what it is, but a laptop will still blow it away. On a computer websites load in fractions of a second while the ipad still takes around ten seconds to load. On a computer you see no checkerboard pattern, scrolling is super smooth, and you never have to wait for video to buffer.
Second, is storage. The maximum you can have is 64gb. And with icloud you only get 5gb. Thats what, 3 hd movies? There is also no external hard drive support, so you have to go completely to streaming and not worry about anything you may have bout until now.
Third is compatibility. The iPad has so many limitations as far as compatible software, accessory ports, and limited file management/file software support. So if you find something you like online, you can't download it, you can't view flash as well a certain kinds of java or direct x software, you cant get music on you device outside of iTunes, you cant have any type cd or DVD support, you're limited to what's in the app store for programs.
Forth is what has been mentioned before, and that is creation. If you want to write a report, it's just much easier to do on a computer, and not just because of the keyboard. On a computer you can have a bunch of resource websites open, have your word proccessor (word, pages, ect.) and just go back and forth between them all so quickly and effortlessly. Sure you could write on the iPad, but it's a lot more trouble.
Don't get me wrong, the iPad is great and can do some things better, like any type of handwriting or hand drawn type programs, it's lighter, smaller, gets better battery life, and is a better ereader than a laptop. But to completely replace your computer it still has some ways to go. If you can only afford one, get a computer unless you specifically need the iPad for a certain use case.