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I get the correct answer for this differential equation

DSolve[{
  D[Y[y], {y, 4}] - 2 p^2 Y''[y] + p^4 Y[y] == Const,
  Y[0] == 0, Y[b] == 0, Y'[0] == 0, Y'[b] == 0
  }
 , Y[y], y]

But if I replace Const with Fn[1] for example the output becomes too huge. 

Fn[q_] := 2/(d*a) Integrate[q*Sin[p x], {x, 0, a}];d=18315;p=Pi n/a;

enter image description here Why is that?

Improve this question
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  • $\begingroup$ Try Const with (173797/5000000000 (a - a Cos[n Pi]))/(a n).DSolve is symbolic solver and likes exact values. $\endgroup$
    – Mariusz Iwaniuk
    Commented Nov 13, 2019 at 18:10
  • $\begingroup$ @MariuszIwaniuk it works, but in future I want to replace Const with a result of function Fn[q_] $\endgroup$
    – Eikthyrnir
    Commented Nov 13, 2019 at 18:21
  • $\begingroup$ Your code does not reproduce the image. I get i.sstatic.net/LrLqL.png. $\endgroup$
    – Michael E2
    Commented Nov 13, 2019 at 19:39
  • $\begingroup$ @MichaelE2 sorry I forgot to add p=Pi n/a in question $\endgroup$
    – Eikthyrnir
    Commented Nov 13, 2019 at 19:54
  • $\begingroup$ @MichaelE2 and I don't know why, but I have Fn[1]=(0.0000347594 (1 - Cos[n Pi]))/n, not 2/18315*... $\endgroup$
    – Eikthyrnir
    Commented Nov 13, 2019 at 20:06

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