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9.  How many poles will be in a pile of telephone poles if there are 30 in the bottom layer, 29 in the 2nd layer and so on until there in one in the top layer?

 Apr 16, 2019
 #1
avatar+18840 
+2

Arithmetic sequence    d = 1

a1 = 30

a30 = 1

 

Sn= n(a1+a30)/2 =  30 (30+1)/2 = 465 poles

 Apr 16, 2019
 #2
avatar+102948 
+2

Just the sum of the integers  1 to 30   =

 

[ 30] * [ 31 ] / 2 =

 

15 * 31  =

 

465 poles

 

 

cool cool cool

 Apr 16, 2019

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