$\displaystyle \sum_{k=1}^n (a_k + b_k) = \sum_{k=1}^n a_k + \sum_{k=1}^n b_k$

$\displaystyle \sum_{k=1}^n ca_k = c \sum_{k=1}^n a_k~~ $ ( $c$ は定数 )

$\displaystyle \sum_{k=1}^n c = nc~~ $ ( $c$ は定数 )