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Golden side ratio (Posted on 2025-05-26) Difficulty: 3 of 5
Show that any triangle has two sides whose lengths a and b satisfy (√5-1)/2<a/b<(√5+1)/2.

No Solution Yet Submitted by Danish Ahmed Khan    
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Solution proof | Comment 1 of 2
Claim:  for all triangles there are two sides, a and b, such that:
   (√5-1)/2<a/b<(√5+1)/2
    0.618   < a/b < 1.618
"suppose not"

 wlog, label the sides so that in length, they are a <= b <= c.
  and wlog scale the triangle so that b = 1.

Now suppose a/b = 0.618 and b/c = 0.618.
Then a = 0.618, b = 1, c = 1.618;   c = a+b  a triangle with zero area.

If the ratios are even smaller, the result is even worse, say a/b is 0.6
Now a = 0.6, b = 1, c = 1.6666;   c > a+b another impossible triangle.

  Posted by Larry on 2025-05-26 14:25:40
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