Good Pattern For Decorate House

I call the upper figure a window , forming by a square with its sides' midpoints joining to each other. Now, I connect 2016 windows side-by-side to make a chain of windows like this:

How many triangles are there in the chain?


Too easy? Try this .


The answer is 40310.

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1 solution

Each window consists of 12 12 triangles; that is, 8 8 small triangles and 4 4 , joining 2 2 of the previous. So, in the chain of windows there are 2016 12 = 24192 2016\cdot 12=24192 triangles, from considering each single window .

When you connect 2 2 windows , you form 6 6 new triangles. As in the chain of windows we have 2015 2015 unions of 2 2 side-by-side windows , the number of triangles resulting from this unions is 2015 6 = 12090 2015\cdot 6=12090 .

Finally, when you connect 3 3 windows , you form 2 2 new triangles. As in the chain of windows we have 2014 2014 unions of 3 3 side-by-side windows , the number of triangles resulting from this unions is 2014 2 = 4028 2014\cdot 2=4028 .

Therefore, adding up the results, the total number of triangles in the chain of windows is 40310 40310 .

I really liked this problem. I hope you create more problems like this which remembers me about fractals. I expect to see soon a harder one, forming weirder and more complicated figures.

As same as me. If we use this method we can easily generalise it to a chain of k windows : 12 k + 6 ( k 1 ) + 2 ( k 2 ) = 20 k 10 12k + 6(k-1) + 2(k-2) = 20k - 10 . Upvoted!

Tran Quoc Dat - 5 years, 2 months ago

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This works for all k 2 k \geq 2 ; for k = 1 k=1 , the answer would be 12 12 .

Shourya Pandey - 5 years, 2 months ago

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Can you give an expression which works for all?

Kushagra Sahni - 5 years, 1 month ago

You could try a harder problem, which we join the windows not in a line but in a square of NxN, or an even harder, MxN rectangle

Tran Hieu - 5 years, 2 months ago

I did the exact same thing... is there any alternative to this?

Ραμών Αδάλια - 5 years, 2 months ago

The approach was same but made one silly error on my first attempt! Took the number of new triangles formed after joining two windows as 8.

Miraj Shah - 5 years, 2 months ago

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