Merging SHA1 values

[whertz]

Does anyone know how to do this:

Let's say I have the SHA1 hash values of a file that is split into two or more.
I also have the SHA1 value of the file when it is complete.
How can I make the SHA1 values of the split files match the hash I have of the complete file? Without using any SHA1 calculations on the files again? (for speed reasons).

[freak]

You can't. These algorithms are designed so this is not possible.

[whertz]

Okay thanks.