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Medium numbers (Posted on 2019-10-31) Difficulty: 3 of 5
How many triples (a,b,c) of integers exist such that a4+b3=c2? [b ≠ 0]

No Solution Yet Submitted by Danish Ahmed Khan    
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Some Thoughts As many as you like | Comment 1 of 2
Since every 6th power is also a cube, write (n*a)^4+(a^2)^3=c^2.

Then LHS = a^4(a^2+n^4). Let (a^2+n^4)=x^2, since then we have a^4x^2, which is square. 

We can also write this as n^4=(x^2-a^2), a difference of squares, so that that at least one solution is guaranteed for every n.

Say n=3, then a^2+81=x^2 has a solution a=40, x=41, with b=120 and c=65600.
Say n=5, then a^2+625=x^2 has a solution a=312, x=313, with b=1560 and c=30468672.

etc.

  Posted by broll on 2019-10-31 09:28:36
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