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# puzzle of the day

Ranch Hand
Posts: 234
one side of the bottom layer of a triangular pyramid has 12 balls. How many are there in the whole pyramid?

Ranch Hand
Posts: 2545

Originally posted by sona nagee:
one side of the bottom layer of a triangular pyramid has 12 balls. How many are there in the whole pyramid?

I will try my best:
The top layer side length is 1 ball, layer sum is 1 ball;
The second layer side length is 2, layer sum is 3;
..................
So: sum = 1 + 2 + ... + side length = 1/2 * ( length + 1) * length;
................
So the twelveth layer has 78 ;
So the grand total is:
1 + 3 + 6 + 10 + 15 + 21 + 28 + 36 + 45 + 55 + 66 + 78 = 364 !

sona gold
Ranch Hand
Posts: 234
great Don.....
wait for a new one .. every day

Ranch Hand
Posts: 210
Hey! Don Liu, how come you have a 3D mind like that! Tell me pls, how long I got to wait for May and Susan?

John Lee
Ranch Hand
Posts: 2545
Something tells me...

Johnson Chong
Ranch Hand
Posts: 210
Don Liu
Calculus of probabilities???!!!
What is that?
I've heard of calculus and probabilities but I just don't think I'll be able to find calculus of probabilities as a mathematical topic.
But I must say what a word.

Wanderer
Posts: 18671
One of the definitions of calculus is "A system or method of calculation". So "calculus of probabilities" just means "calculation of probabilities". As it happens though, serious probability theory has lots of integrals in it, so you definitely need a good grasp of [integral] calculus to understand it.

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