A child is creating a pyramid with building blocks. The top three levels include 5 blocks, 12 blocks, and 19 blocks. How many blocks would be needed for a pyramid 25 levels tall? Use complete sentences to explain how a sum of an arithmetic series was applied.

Question

anonymous · Answer

[5 + (7*0)] + [5 + (7*1)] + [5 + (7*2)] + ... + [5 + (7*24)] = 

(25)(5) + 7(1 + 2 + 3 + ... + 24) =

125 + 7[(24)(25)]/2

anonymous · Answer

That's a lot of blocks for one child!   :-)

anonymous · Answer

The key is that the sum of 1 + 2 + 3 + ... + (n-1) + n = n(n+1)/2

anonymous · Answer

Making sense?  If not, ask any question you like.