i.e. to add:
from 1 to n+1
and n+1 to 1
each column adds up to n+2
so you have (n+1)(n+2) and we have to divide by 2 because we added it twice. (the two sums are multiplied as that is the function of multiplication - to ease addition)
simplifying (0.5)(n+1)(n+2):
=(0.5)(n+2)(n+1)
=(1/2n + 1)(n + 1)
which is correct