Right square pyramid V=189cm³
A)Add 3 and the continue to add 2 more to three each time to get the next number
B)add 1 and add one to the number one each time to get the the next number
The 6 terms of the series are
... 15, 14, 13, 12, 11, 10
The sum of them is 75.
_____
You could make use of the formula for the sum of the terms, or you could work out the sum from the given expression. It is easier just to list the terms and and them up.
