Answer
$\Sigma^{5}_{k=0}k(k-1)$ =$\Sigma^{6}_{i=1}(i-1)(i-2)$
Work Step by Step
When k = 0, then i = 1.
When k = 5, then i = 6.
Since i = k + 1, then k = i − 1.
Thus,
$k(k − 1) = (i − 1)((i − 1) − 1) = (i − 1)(i − 2),
$
and so
$\Sigma^{5}_{k=0}k(k-1)$ =$\Sigma^{6}_{i=1}(i-1)(i-2)$
