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Nested Radical Product (Posted on 2025-03-05) Difficulty: 3 of 5
If

a = √(4 - √(5 - a))
b = √(4 + √(5 - b))
c = √(4 - √(5 + c))
d = √(4 + √(5 + d))

Determine the value of the product abcd.

No Solution Yet Submitted by Danish Ahmed Khan    
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re: Iteration plus guess -- the iterative method | Comment 3 of 4 |
(In reply to Iteration plus guess by Larry)

Iteration could be continued until it settles down (could also use a limit on the absolute value difference instead of inequality):


a=0; prev=-1;
while prev~=a
  prev=a;
  a=sqrt(4-sqrt(5-a))
end
b=0; prev=-1;
while prev~=b
  prev=b;
  b=sqrt(4+sqrt(5-b))
end
c=0; prev=-1;
while prev~=c
  prev=c;
  c=sqrt(4-sqrt(5+c))
end
d=0; prev=-1;
while prev~=d
  prev=d;
  d=sqrt(4+sqrt(5+d))
end
a*b*c*d

a =
          1.32813102610406
a =
          1.44353311628272
a =
          1.45400834933892
a =
          1.45496379259351
a =
          1.45505097726045
a =
            1.455058933229
a =
          1.45505965924739
a =
           1.4550597254999
a =
          1.45505973154574
a =
          1.45505973209745
a =
           1.4550597321478
a =
          1.45505973215239
a =
          1.45505973215281
a =
          1.45505973215285
a =
          1.45505973215285
a =
          1.45505973215285
a =
          1.45505973215286
a =
          1.45505973215286
b =
          2.49721204095683
b =
          2.36262993600061
b =
          2.37149703192152
b =
          2.37092088725225
b =
          2.37095835633226
b =
          2.37095591970525
b =
          2.37095607816053
b =
           2.3709560678561
b =
           2.3709560685262
b =
          2.37095606848262
b =
          2.37095606848546
b =
          2.37095606848527
b =
          2.37095606848528
b =
          2.37095606848528
b =
          2.37095606848528
c =
          1.32813102610406
c =
          1.21836871576504
c =
          1.22732810669003
c =
          1.22659630614774
c =
          1.22665607602821
c =
          1.22665119429492
c =
          1.22665159301265
c =
           1.2266515604472
c =
            1.226651563107
c =
          1.22665156288976
c =
           1.2266515629075
c =
          1.22665156290605
c =
          1.22665156290617
c =
          1.22665156290616
c =
          1.22665156290616
c =
          1.22665156290616
d =
          2.49721204095683
d =
          2.59578576366798
d =
          2.59923937188159
d =
          2.59935988130874
d =
           2.5993640857422
d =
          2.59936423242926
d =
          2.59936423754697
d =
          2.59936423772552
d =
          2.59936423773175
d =
          2.59936423773197
d =
          2.59936423773198
d =
          2.59936423773198
d =
          2.59936423773198
ans =
                        11

Edited on March 5, 2025, 3:43 pm
  Posted by Charlie on 2025-03-05 15:40:16

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