Possible Duplicate:
How do I prove this by induction? (sum of powers of 2)
Summation equation for $2^{x-1}$
How can I prove the following by induction? $ \sum^n_{i=1}2^{i-1}=2^n-1 $ I get to $ \ldots+2^{i-1}=2^n-1+2^{i-1} $ and don't see what the next step is.