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Squares and Triangles (Posted on 2005-01-18) Difficulty: 2 of 5
The following sequence gives a series of square numbers,

10% of 10=1,
20% 0f 20=4,
30% of 30=9,
40% of 40=16,........
90% 0f 90=81,
100% 0f 100=100,.....

Is it possible to create a similar series that gives us triangular numbers as the result?

No Solution Yet Submitted by Juggler    
Rating: 2.8000 (5 votes)

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Solution Generalised Parametric Solution | Comment 5 of 8 |

By conditions of the problem:

(a+rs)(b + rs) = 100*T(n+s), for positive integers a, b, r and t with s=0, 1, 2, ..... where :
T(x) = xth triangular Number = x(x+1)/2

Thus,

ab = 100*T(n) = 50n(n+1)......(i)
(a+r)(b+t) = 100*T(n+1).......(ii)
(a+2r)(b+2t) = 100*T(n+2).....(iii)
(a+3r)(b+3t) = 100*T(n+2).....(iv); and so on

Thus, subtracting (i) from (ii); (ii) from (iii) and (iii) from (iv);
we have:

at + br + rt  = 100(n+1)
at + br + 3rt = 100(n+2)
at + br + 5rt = 100(n+3)

Thus, 2rt = 100, giving  rt = 50, so that:

at + 50b/t = 50(2n+1)........(v)
Or, (at - 50b/t)^2
= 2500(2n+1)^2 - 200ab
= 2500(2n+1)^2 - 10000(n^2+n)
= 2500

Or, at - 50b/t = +/-50.......(vi)

Solving for (v) and (vi), we have:

(at, 50b/t) = (50n, 50(n+1); (50(n+1), 50n)

Or, (50a/r, 50b/t) = (50n, 50(n+1); (50(n+1), 50n)

Or, (a/r, b/t) = (n, n+1); (n+1, n)

Or, (a, b) = (nr, (n+1)t), ((n+1)r, nt), with rt = 50
.........(*)

Consequently, (*) gives the generalised parametric solution to the given problem.


  Posted by K Sengupta on 2007-06-28 11:18:24
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