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Find equal pentagonal and square number

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Shoandeep Radhakrishnan
Shoandeep Radhakrishnan on 9 Sep 2021
Commented: DGM on 14 Sep 2021
A pentagonal number is defined by p(n) = (3*n^2 – n)/2, where n is an integer starting from 1. Therefore, the first 12 pentagonal numbers are 1, 5, 12, 22, 35, 51, 70, 92, 117, 145, 176 and 210.
A square number is defined by s(n) = n2, where n is an integer starting from 1. Therefore, the first 12 square numbers are 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, and 144.
From the above example, the number 1 is considered 'special' because it is both a pentagonal (p=1) and a square number (s=1). Determine the 3rd 'special' number (i.e. where p=s) assuming the number 1 to the be first 'special' number.
Does anyone knows how to solve this cause i am lost

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Accepted Answer

Jan on 9 Sep 2021
This sounds like a homework question. Then the standard way is that you post, what you have tried so fas and ask a specific question.
You can create a list of the pentagonal numbers for the first million natural numbers:
n = 1:1e6;
n2 = n .^ 2;
n5 = (3 * n2 - n) / 2;
Now find the elements which occur in both vectors. See:
doc intersect
doc ismember

More Answers (2)

DGM on 9 Sep 2021
Edited: DGM on 9 Sep 2021
This is a rather brute force method, but it still only takes 1.5ms to find the third match on my dumpster PC.
n = 1:10000;
p = (3*n.^2 - n)/2;
s = n.^2;
ans = 1×3
1 9801 94109401
I imagine a symbolic solution might be more elegant, but I'll leave that to someone who uses the symbolic toolbox more than I do.

Jan on 10 Sep 2021
Edited: Jan on 10 Sep 2021
And a two-liner:
n = 1:10000;
find(any(((3*n.^2 - n)/2) == (n.^2).'))
Matlab can be very elegant. Do you understand the details of this solution?

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