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I try to calculate something like 1.569732312*Pi and by default mathematica returns something like 4.93146 (only 5 digits after floating point), but I believe there is much more digits, especially because Pi has infinite number of digits.

if I do like this

N[1.569732312*Pi, 50]

It still return the same 5 digits 4.93146

As I understand from documentation, I have to use Rationalize[]. If I do like this

N[Rationalize[1.569732312]*Pi, 50]

result is the same 4.93146

and only if I remove 3 digits like this

N[Rationalize[1.569732]*Pi, 50]

it gives me all digits like this

4.9314585193048133197984421233221006214165911156199

What I am doing wrong and how to calculate 1.569732312*Pi with 50 digits precision?

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  • $\begingroup$ 1.569732312`50*Pi - note the backtick. See also here. $\endgroup$ – corey979 Oct 31 '16 at 22:18
  • $\begingroup$ Thanks a lot! Yes, if I put something like 1.569732312`50*Pi, it works fine. $\endgroup$ – Zlelik Oct 31 '16 at 22:32
  • $\begingroup$ 1.569732312 is a machine double. What 50 digit result would you expect to get from that product? (If you want a 50 digit number that has 1.569732312 followed by decimal zeroes, the tactic of using 1.569732312`50 as proposed by @corey979 is appropriate). $\endgroup$ – Daniel Lichtblau Oct 31 '16 at 22:33
  • $\begingroup$ I want to know the result of my equation. Like Pi=3.1415926...., If I multiple it by 2, it will be 6,2831852, but I want to know all 50 digits of this equation 2*Pi, because Pi has infinite digits. and later I want to change 2 to some different number. $\endgroup$ – Zlelik Oct 31 '16 at 22:49
  • $\begingroup$ you should review the details section in the docs for Rationalize to learn whats happening there. $\endgroup$ – george2079 Nov 1 '16 at 0:55
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It is about backtick. This example works fine

1.569732312`50*Pi
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    $\begingroup$ Similarly, 2.0`50*Pi gives 2*Pi to 50 dgits $\endgroup$ – bill s Oct 31 '16 at 23:16

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