Author: Unknown
Subject: Lower triangular matrix
Date: Tuesday, 03 Mar 2020, 17:09:21

Could you explain the meaning of the "(n-j+2)" term in the "n + (n-1) + ... + (n-j+2)" line of the lower triangular matrix problem solution?

Also, could you explain how you go from this
n + (n-1) + ... + (n-j+2)
to this
(2n-j+2)/2 * (j-1)
to this
(first term + last term)/2 * #term?

Thanks