In a math text I have an equation:
S = $\sum \limits_{0 \leq k \leq n} (a + bk)$
By the communitive law we can replace $k$ by $n - k$, obtaining
S = $\sum \limits_{0 \leq n-k \leq n} (a + b(n - k)) = \sum \limits_{0 \leq k \leq n}(a + bn -bk))$
In the last equation how does the index $0 \leq n - k \leq n$ become $0 \leq k \leq n$ again?