Showed this maths trick to Rachel when I was in the Countdown audience recently :
RR to players:
Start with any two numbers, say 7 and 1, and write them in a column.
Add them to get 8, and put 8 in the third column.
Add 1 and 8 to get 9, put in 4th column.
8 + 9 = 17, so put 17 in the 5th column.
9 + 17 = 26, so put 26 into the 6th column.
Repeat this same process until you have 10 numbers in the column; 7, 1, 8, 9, 17, 26, 43, 69, 112, 181. (An example of a generalised "Fibonacci" sequence)
But .... before the 10th number has been entered, RR writes 473 on a card, lets the players sum the 10 numbers together, and
those selfsame 10 numbers add to 473 !
Start with any other two numbers, say 1 and 1 (THE Fibonacci sequence) :
1, 1, 2, 3, 5, 8, 13, 21 .....  I can say the total of the first 10 numbers is 143 without writing the whole list.
To the mathematicians among you, how and why does this trick work ?
Sum of 10 numbers
Moderator: Michael Wallace

 Acolyte
 Posts: 113
 Joined: Sat May 31, 2014 4:39 pm
 Location: Bournemouth
Re: Sum of 10 numbers
I've spotted a link between the 7th number of the sequence being multiplied by 11 to give the sum of the first ten numbers. That is the how. I'm stuck on the why though!
 Mark Kudlowski
 Enthusiast
 Posts: 433
 Joined: Fri Sep 12, 2008 3:15 pm
Re: Sum of 10 numbers
The proof is on this document : (Paste the URL)Sean Fletcher wrote:I've spotted a link between the 7th number of the sequence being multiplied by 11 to give the sum of the first ten numbers. That is the how. I'm stuck on the why though!
http://mkhometuition.co.uk/index_files/ ... _Proof.pdf
Top left of the front page graphic
Full details on Example(2) on Page 3
Last edited by Mark Kudlowski on Thu Jul 24, 2014 9:38 pm, edited 5 times in total.

 Acolyte
 Posts: 113
 Joined: Sat May 31, 2014 4:39 pm
 Location: Bournemouth
Re: Sum of 10 numbers
That seems really obvious now I know the why. Cool trick!
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