An octagonal number is a number that is represented by the set of figures given below.
A certain octagonal number is the sum of the squares of three terms in an arithmetic sequence with a common difference of 704. What is that octagonal number?
(In reply to
For starters by e.g.)
This UBASIC program uses e.g.'s expressions for the square sum and the octagonal numbers:
list
10 for N=1 to 1000000
20 V=3*N*N2*N2*704*704
30 if V@3=0 and V>0 then
40 :M=int(sqrt(V/3)+0.5)
50 :if M*M=V/3 then
60 :print M,3*m*m+2*704*704
70 next
OK
run
2836 25119920
OK
The result indicates the octagonal number is 25,119,920 and the middle term in the arithmetic sequence is 2,836.
25,119,920 is the 2,894th octagonal number, based on the formula e.g. found.

Posted by Charlie
on 20060613 11:07:39 