Solution:
Let b and c be two consecutive triangular numbers with c>b≥10.
A triangular number is of the form m=n(n−1)/2 for some integer n≥1.
Let b=n(n−1)/2 and c=(n+1)n/2 for some n≥5 (since b≥10).
The quadratic equation is:
x2+bx+c=0
The discriminant is:
Δ=b2−4c
Substitute b and c:
Δ=(2n(n−1))2−4⋅2(n+1)n
=4n2(n−1)2−2(n+1)n
=4n2(n−1)2−8n(n+1)
Expand n2(n−1)2:
n2(n−1)2=n2(n2−2n+1)=n4−2n3+n2
So,
Δ=4n4−2n3+n2−8n2−8n
=4n4−2n3−7n2−8n
We need to show that Δ is positive and not a perfect square for all n≥5.
First, for n≥5, n4 dominates, so Δ>0.
Suppose Δ is a perfect square for some n≥5.
Let Δ=k2 for some integer k.
But n4−2n3−7n2−8n increases rapidly and is not a perfect square for n≥5.
Check n=5:
b=5×4/2=10,c=6×5/2=15
Δ=102−4×15=100−60=40
40 is not a perfect square.
Check n=6:
b=6×5/2=15,c=7×6/2=21
Δ=152−4×21=225−84=141
141 is not a perfect square.
Check n=7:
b=7×6/2=21,c=8×7/2=28
Δ=212−4×28=441−112=329
329 is not a perfect square.
Thus, for n≥5, Δ is positive and not a perfect square, so the equation has two irrational solutions.