Cracked cement or asphalt is usually repaired with tar, or torn up and replaced. Once a slab is cracked, water and plants will continue to break it up. There isn't much you can do that I'm aware of.
The easy way to fix is to simply pour concrete in the crack and smooth it. However this is a temporary fix that could re-crack. The better solution is to remove the old one and pour a new one on a more solid foundation.
