226. Calculate the sum of the coefficients 1) of the expansion $(a+b)^{n}$, 2) of the expansion $\left.(a-b)^{n}, 3\right)$ the sum of the coefficients of the terms occupying even positions in the expansion $(a+b)^{n}$.
226. 1) Let $a=b=1$. The sum of the binomial coefficients will be $(1+1)^{n}=2^{n}$.
2) Let $a=b=1$. The sum of the coefficients $(1-1)^{n}=0$.
3) The sum of the binomial coefficients in even positions, denote it as $S_{p}$, the sum of the coefficients in odd positions as $S_{i}$; from 1) and 2) it follows that $S_{p}+S_{i}=2^{n}, S_{p}-S_{i}=0$, hence $S_{i}=S_{p}=2^{n-1}$.