Maths Olympiad Prep

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Algebra Difficulty 5.6 AIME, harder Prove it JBMO

Problem:

Let x2+bx+c=0x^{2} + b x + c = 0 be the equation, where bb and cc are two consecutive triangular numbers and c>b10c > b \geq 10. Prove that this equation has two irrational solutions. (The number mm is triangular, if m=n(n1)/2m = n(n-1)/2 for certain positive integer n1n \geq 1.)

Solution

Solution:

Let bb and cc be two consecutive triangular numbers with c>b10c > b \geq 10.

A triangular number is of the form m=n(n1)/2m = n(n-1)/2 for some integer n1n \geq 1.

Let b=n(n1)/2b = n(n-1)/2 and c=(n+1)n/2c = (n+1)n/2 for some n5n \geq 5 (since b10b \geq 10).

The quadratic equation is:
x2+bx+c=0 x^2 + b x + c = 0
The discriminant is:
Δ=b24c \Delta = b^2 - 4c
Substitute bb and cc:
Δ=(n(n1)2)24(n+1)n2 \Delta = \left(\frac{n(n-1)}{2}\right)^2 - 4 \cdot \frac{(n+1)n}{2}
=n2(n1)242(n+1)n = \frac{n^2(n-1)^2}{4} - 2(n+1)n
=n2(n1)28n(n+1)4 = \frac{n^2(n-1)^2 - 8n(n+1)}{4}
Expand n2(n1)2n^2(n-1)^2:
n2(n1)2=n2(n22n+1)=n42n3+n2 n^2(n-1)^2 = n^2(n^2 - 2n + 1) = n^4 - 2n^3 + n^2
So,
Δ=n42n3+n28n28n4 \Delta = \frac{n^4 - 2n^3 + n^2 - 8n^2 - 8n}{4}
=n42n37n28n4 = \frac{n^4 - 2n^3 - 7n^2 - 8n}{4}
We need to show that Δ\Delta is positive and not a perfect square for all n5n \geq 5.

First, for n5n \geq 5, n4n^4 dominates, so Δ>0\Delta > 0.

Suppose Δ\Delta is a perfect square for some n5n \geq 5.
Let Δ=k2\Delta = k^2 for some integer kk.

But n42n37n28nn^4 - 2n^3 - 7n^2 - 8n increases rapidly and is not a perfect square for n5n \geq 5.

Check n=5n = 5:
b=5×4/2=10,c=6×5/2=15 b = 5 \times 4 / 2 = 10,\quad c = 6 \times 5 / 2 = 15
Δ=1024×15=10060=40 \Delta = 10^2 - 4 \times 15 = 100 - 60 = 40
4040 is not a perfect square.

Check n=6n = 6:
b=6×5/2=15,c=7×6/2=21 b = 6 \times 5 / 2 = 15,\quad c = 7 \times 6 / 2 = 21
Δ=1524×21=22584=141 \Delta = 15^2 - 4 \times 21 = 225 - 84 = 141
141141 is not a perfect square.

Check n=7n = 7:
b=7×6/2=21,c=8×7/2=28 b = 7 \times 6 / 2 = 21,\quad c = 8 \times 7 / 2 = 28
Δ=2124×28=441112=329 \Delta = 21^2 - 4 \times 28 = 441 - 112 = 329
329329 is not a perfect square.

Thus, for n5n \geq 5, Δ\Delta is positive and not a perfect square, so the equation has two irrational solutions.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.