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250 toothpicks how many rows (pyramid) could you make

 Jan 4, 2016

Best Answer 

 #3
avatar+130466 
+10

Very nice, Melody......in a slightly different manner.....the sum of the first n integers  =

 

[n (n + 1)] / 2  <=  250

 

n^2 + n <= 500

 

n^2 + n - 500 <=  0

 

On the graph here :  https://www.desmos.com/calculator/ajecyupfie , the largest positve root  = about 21.87....so,  we can take the floor of  21.87  as the answer   =  21 rows

 

 

cool cool cool

 Jan 5, 2016
 #1
avatar+2499 
0

i think 250/8 = 31.25  (31)

 Jan 4, 2016
 #2
avatar+118696 
+10

Hi Solveit and Guest ,

 

250 toothpicks how many rows (pyramid) could you make

 

1 on the top

2 on the next row

3 on the next row  etc

 

1+2+3+4+......+n250 

1+2+3+....+n 

is the sum of an AP

a=1d=1Sn=n2(a+L)Sn=n2(1+n)son2(1+n)250n(1+n)500n2+n5000Solve for 0n=1±1+20002n=1±20012n is not negativen=1+20012n=21.8so there can be 21 rows

 

 

231 toothpickswill be used andthere will be 19 left over :)

 Jan 4, 2016
 #3
avatar+130466 
+10
Best Answer

Very nice, Melody......in a slightly different manner.....the sum of the first n integers  =

 

[n (n + 1)] / 2  <=  250

 

n^2 + n <= 500

 

n^2 + n - 500 <=  0

 

On the graph here :  https://www.desmos.com/calculator/ajecyupfie , the largest positve root  = about 21.87....so,  we can take the floor of  21.87  as the answer   =  21 rows

 

 

cool cool cool

CPhill Jan 5, 2016
 #4
avatar+33654 
+5

Suppose the toothpicks are arranged as follows (i.e. 3 per row; after all they are long and thin!):

 

pyramid

Then you get 12 rows with 16 left over.

 Jan 5, 2016
 #5
avatar+118696 
0

That is great Alan.  I bet that is exactly what was intended.

 

I did think my pyramid was very weighed down (flattened).  Maybe with time.  LOL

Then again maybe my toothpicks were standing upright.  That would have made a very tall skinny 'tower' I guess. :)

 Jan 5, 2016
edited by Guest  Jan 5, 2016

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