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avatar+49 

Evaluate the sum 6321+6521+6721+6921+

 Feb 20, 2019

Best Answer 

 #2
avatar+26396 
+4

Evaluate the sum

6321+6521+6721+6921+

 

6321+6521+6721+6921+=6(31)(3+1)+6(51)(5+1)+6(71)(7+1)+6(91)(9+1)+=624+646+668+6810+=6(124+146+168+1810+)=6k=112k(2k+2)=6k=114k(k+1)=64k=11k(k+1)=32k=11k(k+1)|1k(k+1)=1k1k+1=32k=1(1k1k+1)=32[k=1(1k)k=1(1k+1)]=32[1+k=2(1k)k=1(1k+1)]=32[1+k=2(1k)k=2(1k)]=32(1+0)=32

 

laugh

 Feb 21, 2019
edited by heureka  Feb 22, 2019
edited by heureka  Feb 22, 2019
 #1
avatar+6251 
+1

A little toying with this and lookup at OEIS show that this sum is equal to34k=1 (k+12)1=32

.
 Feb 20, 2019
 #2
avatar+26396 
+4
Best Answer

Evaluate the sum

6321+6521+6721+6921+

 

6321+6521+6721+6921+=6(31)(3+1)+6(51)(5+1)+6(71)(7+1)+6(91)(9+1)+=624+646+668+6810+=6(124+146+168+1810+)=6k=112k(2k+2)=6k=114k(k+1)=64k=11k(k+1)=32k=11k(k+1)|1k(k+1)=1k1k+1=32k=1(1k1k+1)=32[k=1(1k)k=1(1k+1)]=32[1+k=2(1k)k=1(1k+1)]=32[1+k=2(1k)k=2(1k)]=32(1+0)=32

 

laugh

heureka Feb 21, 2019
edited by heureka  Feb 22, 2019
edited by heureka  Feb 22, 2019
 #3
avatar+130466 
+1

Very nice, Heureka......I wondered if there was a way to determine this "by hand"

 

 

cool cool cool

CPhill  Feb 21, 2019
 #4
avatar+2234 
+1

Hello Heureka,

In your solution, one of the (k) factors seems to have randomly evaporated:   

 

32k=11kevaporated?(k+1)|1k+1=1k1k+132k=1(1k1k+1)

 

I thought you would like to fix this fuckuplaugh

 

 

GA

GingerAle  Feb 21, 2019
 #5
avatar+26396 
+3

Sorry GA,

 

I corrected my typo:

1k(k+1)=1k1k+1

 

laugh

heureka  Feb 22, 2019
edited by heureka  Feb 22, 2019
 #6
avatar+26396 
+3

Thank you CPhill,

 

laugh

heureka  Feb 22, 2019

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