A quadratic question
July 8, 2006 pm31 4:13 pm
This one has been puzzling me: for ax2 + bx + c = 0, let a, b, and c be integers between -100 and 100 inclusive, a different from 0.
How many of these equations have a double root (ie, b2 – 4ac = 0).
All I can come up with is a brute force count, which I really don’t want to do. Anything clever out there?
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So, two cases: if b is 0, we must have ac = 0. Since a isn’t 0,
c must be, so that’s 200 equations we pick up there.
Otherwise, b isn’t zero. Certainly b has to be even. So let b
= 2k, where k runs from -50 to 50, skipping 0. Obviously, the
sign isn’t going to matter here, so we can just take k running
from 1 to 50 and double our answer at the end. Then we have k^2
= ac. So, we need the number of factorizations of 1^2, 2^2,
3^2, …, 50^2 into positive integers. Then we double that
(because we can take negatives of both a and c) and then we
double the result again (because I said so earlier, for the sign
of b) and add the 200 in from the other case. Now, a
factorization into two parts is the same as a factor, so we need
the sum of the number of factors of the first 50 squares.
Unfortunately, I have no clever way of approaching that problem
at all. Mathematica tells me that the first fifty squares have
the following numbers of factors: {1, 3, 3, 5, 3, 9, 3, 7, 5, 9,
3, 15, 3, 9, 9, 9, 3, 15, 3, 15, 9, 9, 3, 21, 5, 9, 7, 15, 3,
27, 3, 11, 9, 9, 9, 25, 3, 9, 9, 21, 3, 27, 3, 15, 15, 9, 3, 27,
5, 15}, for a total of 470, which means that 2080 of the total
8080200 such equations have a double root, 0.0025…%.
Sorry JBL, the spam filters grabbed your comment, and I only just found it today.
Thanks for the reasoning. Of course the |100| requirement was arbitrary. If we let 100 get big, the contribution from your first case gets small, and it looks like the percentage goes to 0…
Jonathan