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Thank you for answering.
Then my Student has to be a double and not a float.
The problem is that my Student is a float and not a double. I can't redefine it as a double. It changes all my results.
Have you an idea of what i can do?

You can extend it from float to double. Float precision values can be exactly represented as double precision ones.

Thank you for all!
I just have noticed my problem.
The studenttdistribution function that I use is the one that gives me the One tailed P- Value and i need the two tailed P- Value.
Thank you very much!