You are MIguel Cervantes de Navas y Colon, captain in the Royal Spanish Army in Sevilla in the year 1842. Outside your barracks window, there is s a stack of cannnonballs, as shown . On an idle afternoon you decide to calculated how many cannonballs are in the stack.. What is the number of cnnonballs?
a. 347
b.361
c.285
d.385

Question

You are MIguel Cervantes de Navas y Colon, captain in the Royal Spanish Army in Sevilla in the year 1842. Outside your barracks window, there is s a stack of cannnonballs, as shown . On an idle afternoon you decide to calculated how many cannonballs are in the stack.. What is the number of cnnonballs?
a. 347
b.361
c.285
d.385

clamin · Answer

picture is here https://www.google.com/search?q=cannonballs&rlz=1C1CHFX_enUS590US590&espv=2&biw=946&bih=694&source=lnms&tbm=isch&sa=X&ved=0CAYQ_AUoAWoVChMIwKaYl9OqyAIVg5yACh2IzQ3u#tbm=isch&q=miguel+cervantes+de++naas+y+colon+cannonballs+stack&imgrc=Z87sKEzUfiK4EM%3A

directrix · Answer

http://openstudy.com/study#/updates/537db367e4b0e90e4b2dcc85 may help, especially for the diagram. I counted 10 layers in the stack. Each stack contains a perfect square of balls. 10^2 + 9^2 + .... + 1^2 = (10 * 11 * 21) /6 = ?

directrix · Answer

P(n) = n(n + 1)(2n + 1)/6  is the formula for the sum of the first n perfect squares. It can be proved by mathematical induction.

@clamin