Precision
 Usage
• Precision[x] 给出数x的精度数位的数目。
Notes
• 如果x是一个数，对出现在x中的所有数字Precision[x]给出Precision的最小值。 • 当使用到如同整数的精确数时，Precision给出Infinity。 • 对机器精度数，Precision给出$MachinePrecision。 • 参见 Mathematica 全书 : 节 3.1.4.
 Further Examples
Here is an approximate real number.
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This gives the total number of digits entered to specify the real number.
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This evaluates  using numbers with 30 digits of precision.
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The result has 30 digits of precision.
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Mathematica treats 3.0 as a machine-precision number.
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Giving anything less than $MachinePrecision digits yields a machine-precision number.
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This evaluates  using 30-digit precision numbers.
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In this case, the result has a precision of exactly 30 digits.
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If you give input only to a few digits of precision, Mathematica cannot give you high-precision output.
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If you want Mathematica to assume that 0.142 is exact, then you have to show this explicitly.
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