How to derive the worst case time complexity of Heapify algorithm?
It's a variable substitution.
First, realize that in the leftmost side of the equation, the last term of the sum is zero (because when $i = k$, $k-i = 0$). So, the range of the first summation can be written as $1 \le i \le k-1$. Now, substitute $i$ with $k-i$. $i$ iterates over the set ${1, 2, ... , k-1}$ and $k-i$ iterates over the set ${k-1, ... 2, 1}$, they are the same set, so, we can do this.