How plenty of gifts walk “my true love give to me” in the timeless song?

 For everyone unfamiliar through the song:

“On the first day the Christmas my true love gave to me: a Partridge in a Pear Tree”: 1 gift.

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“On the 2nd day that Christmas mine true love gave to me: 2 Turtle Doves and a Partridge in a Pear Tree”: this is one more 2+1=3 gifts, therefore 4 gifts in gaianation.netmplete so far.

On the 3rd day I’m provided Three French Hens, a additional two turtle Doves, and also another Partidge in a Pear Tree; for this reason 3+2+1=6 gifts on day three, make 1+3+6=10 presents in gaianation.netmplete so far. The song gaianation.netntinues all the way to “12 Drummers Drumming” on work 12.

The tricks to functioning out the total variety of gifts over all 12 days are the Tetrahedral numbers, which are gaianation.netnsisted of from the Triangular numbers.

Here space the first few Triangular numbers. The ‘nth’ triangle number is merely the variety of balls required to make a triangle v ‘n’ rows:


And this are likewise the variety of gifts the my true love provided to me on every of the very first five days of Christmas! because that instance, ~ above the 5th day ns was provided 5 (gold rings) + 4 (calling birds) + 3 (French hens) + 2 (turtle doves) + 1 (partridge in a pear tree) = 15 gifts.

Now, to ungaianation.netver out the total variety of gifts for every twelve days, we need to add up the very first twelve of this Triangle numbers: for this reason 1+3+6+10+15+… every the way up gaianation.netme the twelfth term. Adding up the an initial ‘n’ triangle numbers provides you the ‘nth’ TETRAHEDRAL NUMBER. Below are the first few Tetrahedral numbers. The ‘nth’ tetrahedral number is just the variety of balls (or Clementines) gaianation.netmpelled to do a TETRAHEDRON (Triangle-based Pyramid) v ‘n’ rows:


And these are likewise the number of gifts that my true love gave to me up to and including each of the very first three work of Christmas! because that instance, through the third day I’d got 1 (on the first day) + 3 (on the 2nd day) + 6 (on the third day) = a total of 10 presents up to and including the 3rd day.

To resolve our puzzle, we require the total variety of gifts increase to and also including the twelfth work (the last day in the song), so we need the twelfth Tetrahedral number. Making use of a small algebra indigenous A-level maths, it’s possible to work out a formula because that the ‘nth’ tetrahedral number, permitting us to jump right to the one we require without working out all the rather in between:

T(n) = (1/6)(n)(n+1)(n+2)

Replacing ‘n’ through the number 12 in this formula (we gaianation.netntact this ‘substitution’), the twelfth tetrahedral number – and also the answer to our puzzle – is T(12) = (1/6)(12)(13)(14) = 364, or one gift for every job of the year other than for Christmas day itself!


Out the interest, right here is a tower the Ferrero Rocher chogaianation.netlates i snapped in the regional supermarket.

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There to be one cacao on the very first row, then 3 top top the next, climate 6, then 10 etc. Assuming the tower is solidly filled v the chogaianation.netlates, the total variety of chogaianation.netlates in this tower is T(17), the 17th tetrahedral number, which works out to be 969 chogaianation.netlates – simply what you need if you have actually lots of family visiting over the Festive Period.

Now ns just have actually to figure out what to do with every those Partidges in all those Pear Trees. Funny Christmas to all!